Technical small block knocking sound on cold start, balancer?

I don't post much here, hopefully this is in the right spot.

I have a small very mild (probably not even 200hp) small block 350 in a 33 Chevy truck. It's a crate motor that was new about 15 years ago, with less than 10k miles on it. It's new to me as of this month, so I'm still learning it.

There's an intermittent random knocking sound that I've started noticing a week ago or so when I start the motor in the morning when the motor is dead cold. Sounds to me like its coming from the harmonic balancer. I move my head all around the motor and put my hands on anything that's not moving, and it definitely sounds like it's coming from the balancer.

Goes away after a minute or two, and I don't hear it again until another dead cold start in the morning. "cold" here is 55-60 degrees in the morning. The motor runs fine, and I don't think I feel any abnormal vibrations.

Oil is full, belts are tight, and I don't see or feel anything on the outside of the motor that is physically knocking against something else.

Is there any way to verify if it's the balancer or not? Remove the balancer and belts and start it up? Anything else I should try to eliminate?



Tried to get a video of it this morning to share what the sound is like, but of course as soon as I hit record, the noise stopped. So the solution might be to put a microphone near the engine and the noise will never happen :-D

 

Mechanical fuel pump? Wonder if there is something going on there....noisy until pressure is built up....?

 

Typically, that is loose main bearings, it quits when oil pressure builds up to give it a cushion. Is it a heavy sounding thud type knock?





Bones

 

Really sounds like a wrist-pin knock. Had one do it for years at first start of day. After that it was gone. Drove it ten years and never had a problem. Always had good oil pressure. If you really believe it to be the balancer, remove bolt, add a lock washer and tighten down before starting. That should hold it tight unless balancer is worn inside.

 

Had that sound on a 472 Cad motor. Turned out to be the aftermarket torque converter bolts were too long and bottomed out before the converter was tight to the flywheel. So the converter was loose. Put in shorter bolts and the noise was gone.

 

Yes, mechanical fuel pump on the passenger side. I can't recall if I put my hand on that or not, I'll make sure and do that tomorrow morning.

The sound isn't a deep thud sound, it's kind of a medium pitched knock, kind of like knocking on a solid wood door sharply with your knuckle, but a higher pitched.

If I drop the pan, will I have access to check the main bearings with the engine still mounted to the frame? I've never really done any engine building before, but I'm decent at wrenching on my back.

It's a TH350 automatic transmission (the only vehicle with an automatic that I own!). But the sound is definitely not coming from the torque converter.

 

Forged pistons will make a little rattle or slight “ piston knock” until they warm up.

 

I don't think there's an inspection plate on the bell housing, it's just open, I can see the flywheel (I'm not used to automatics, is this normal?) I'll take a look under there and see if anything looks cracked.

I don't know for certain, but I'm pretty sure the pistons aren't forged. The engine isn't a performance motor, I think it's a 190hp goodwrench crate motor. I have a receipt for it from 2005, part# 10067353

 

I don't see any cracks in the flywheel/flexplate, and the bolts don't appear to be to long. I can't see the top of it though. Can I just put it in neutral and bump the starter to turn the flex plate? Or turn the crank with a breaker bar?

There is definitely nothing in the way of the oil pan, so if there's anything I can tighten on the underside, I'll pull the pan next time I change the oil. Bummer, as I JUST changed the oil today. haha!

Should I get a cover for the bell housing? Or is it ok to leave it open? I do intend on driving it alot, but only on paved roads and never in the rain.

 

I rotated the crank all around 360 degrees stopping to look at the flex plate, using a light and a dentist mirror to help. Don't see any cracks, or any bolts that are sticking out to far.

I'm going to pull the belts and see if any of the pullys are loose. I doubt it, but it's easy to check.

 

Main bearings have a very deep “ thud knock” if it’s a lot sharper than that, it’s probably not mains. Also with 10,000 miles on a new engine, something mighty strange would have to have happened to have bad main bearings. They are usually real tuff.




Bones

 

I thought only Fords had this issue.
I had a OT work truck with a 351, made a knock for about 20 seconds when first started ... my mechanic also had a Ford and said his does the same "the engines come from the same Canadian plant"

I drove it hard for over 12 years, sold it with a bad transmission and ran well.
But yeah, in a hot rod I would want to try and correct the

 

It's not a deep sound.

 

Get a wooden dowel rod or a round metal rod. Hold one end against the little flap on your ear. Touch different places on the motor to see where it sounds loudest. Harbor Freight sells a cheap stethoscope if you don't have a dowel or rod. Try the side of the block if you can't find anything else.....could be piston slap. Be careful not to put the rod too close to any moving parts.....

 

Is your 350 a vortec 350 ? if so they had a lot of piston slap when cold, so much so it was called vortec rattle
It wasn’t harmful lots of vortec motors with 200 k or more on them

 

X2 on wrist pin knock. Had a 350 in my Nomad that did it. Only when cold, first start up, went away after a minute or so. In my case, turned out to be the wrist pin on #6 cylinder.

 

I've got one of those stethoscopes somewhere, got it along time ago, I'll dig that out. Totally forgot about that, haha! Wonder what else I've forgot :-\

I don't think it's a vortec, but I don't know for sure. Any way to verify?

I don't even know what wrist pin knock is, I've never done any internal motor work except for cam swaps. I'll try to find my stethoscope and see if the noise is louder at one end of the block or the other. As much as I'd like the noise to go away, I'd like it to stay for long enough to trace it down! hah! It goes away after a minute or so, and then I can't check again until the next morning. Maybe if I had a freezer large enough to put the truck into.... :-D



On a side note, I did change the oil today, replaced the spark plugs (probably not needed, since there's not many miles on the motor, but plugs are cheap...) and I set the initial timing to 10+ on the balancer (I believe that's correct?), it was set to zero at idle with the vacuum advance plugged (didn't make a difference, no vacuum pull at idle anyway). It does seem to run smoother now. I wonder if that will affect the knocking, if indeed it's and internal noise? I did a quick run down the freeway for the first time, it cruises nicely at 75mph. at almost 60 it's 2,000 RPM, but couldn't bother to look when I was going faster. It does pull pretty darn good from 75 to 90, much quicker than I would have guessed, since it can't even break the skinny old tires loose from a stop. But I guess that's what you get in a fairly light weight vehicle with a V8, even if it doesn't make alot of power and the body is aerodynamic like a cinder block. It does need overdrive for the freeway though. I'll fix that later with a manual transmission.

Another side side note, I greased the brake pedal linkage, cleaned and lubed the throttle linkage, and it's feeling much smoother at the pedals.

 

A piece of hose works really good for isolating noises, hold one end to your ear and move the other end around different placers on the engine while it is knocking, you should be able to isolate the general area like this.

 

No one has mentioned an exhaust leak.... I had one on a 312 Ford years ago that would “ click” while cold, but shortly after the truck started the manifold would heat up, expand, the click would go away.......could be a possibility???






Bones

 

I vote wrist pin or skirt slap .....

 

Could be the water pump also. Take the belts off and fire it up, see what happens. With any luck, ( if you want to call that luck) that's all it is. Either way, I'm hoping for an easy fix for 'ya!

 

I see the post suggesting using a dowel or similar to find the noise location, my Dad had a 3 foot long screwdriver he used for that, worked real well. He said the screw driver was one he used as a civilian air craft mechanic during WW2.

 

Great for whapping Crew Chiefs on the head when they needed "adjustment" too I betcha.

 

If it’s not a vortec maybe gm used the same pistons in the crate motors. Vortec 350 was used in trucks from 96 to 99? , your motor is a little newer did gm recycle the piston design?

 

O'Reilly's and Autozone also sell inexpensive automotive stethoscopes.


On the Flexplate/flexplate bolts. AS soon as you start it up. Put your foot on the brake, drop it in gear and put it under a bit of a load and see if the knock goes away. Usually under load flexplate and converter bolt noises will go away until you take the load off again.

 

Ok, managed to get a pretty good audio clip of the sound. In real life it's not quite as tinny sounding, and normally it's a random clicking at idle, but by time I had stepped out of the truck and got my phone out, it had stopped at idle already. So I opened the throttle a bit while filming and it started sounding rapidly like that. It's only bearly loud enough to hear while inside the truck with the windows open.

 

Well, that’s not a knocking sound. Don’t remember all that has been done or suggested, but I would pull the belts off and see if the noise goes away.

Over my years I have heard belts make every kind of noise there is!







Bones

 

That little cklickity click could be a cracked pulley ,either fan or crank , likes been mentioned , pull the belts and have a listen