Okay, I've let this noise, coming from my engine, mess with my head for a few days and now it's time to share my pain. About a week and a half ago, my 305 SBC started making a noise. Kinda like a "knocking" noise. So, I first thought it was carbon build up, as it had done that a year ago. Like a year ago, I ran some Seafoam through the engine, but didn't quiet the noise. I pulled the plug boots off in an attempt to find the source and still nothing. It sounds like it's coming from the valve train, but pulling the valve covers revealed nothing. All the while, the engine is running great. Idles where it should and accellerates great. I checked for loose flywheel/converter bolts. All good. I noticed that when I cruise and remove my foot off the gas, the noise all but dies. It gets that quiet, but it's still there. I looked for the fan making contact with something... nada. I checked the fuel pump rod... nothing. Today, I ran the engine and drove a bit and it now sounds like a "clanging" sound as well as the "knock". My next thing to check, will be the timing chain, but I can't see how that could be it. I'm just desperate at this point. So, any ideas for me to try before I get into a full blown rebuild? Any and all ideas are welcome.
fred, check and see if the flexplate itself is cracked,its sounds like the noise is happening under load, does the noise quiet down if you rev the motor while parked or just when your moving ,how many miles on the engine and what year is it. mark
When it's parked and idleing and I rev it, the noise gets louder. It's an '84 305 with about 30-40K miles on it. I did swap trannys about 2 months ago. When driving, it'll quiet down if I take my foot off the gas. It's quiet when ideling.
I had a noise in a 350 that was very similar to what you describe and it did turn out to be a cracked flex plate. It was cracked at the bolt circle where it meets the crank flange and it extended almost 180 degrees around the the center of the flex plate.Drove me nuts.
i dought its the timing chain with that amount of miles,possible but not likely.and since you did the trans recently i would definitly look at the flexplate- between the large holes and around where it bolts to the crank. i know its not easy to see all the way up there,you'll have to pull the inspection cover and the starter down to get the best view.
Good ideas on the flexplate. It's a Tbucket, but I'll figure out a way to get it in the air enough to get under it.
I agree with checking the flex plate. The hard part is seeing the cracks with it bolted up and actually knowing that's what it is before you have it in your hand.
unbolt the converter and shove it into the trans and tie it back with a coathanger. then ck to see if the noise is gone with the engine running/revving
Son had flex plate bolts work loose on 350 in El Camino. Made those sounds. After bolts worked loose again he removed flex plate. Cracked badly. Maybe the cracks and flexing kept working his bolts loose. Does sound like the flex plate.
Water Pump? I should ask if you have a fan on the water pump of an electric fan. I am asking because I had a truck that did something similar and it turned out that the fan was deflecting and getting out of ballance at higher RPM's and causing the WP to rattle.
I know it might take some doing, especially in a T bucket with limited room underneath, but if the visual inspection of the flex plate doesn't tell you anything you might want to put a new one on just for giggles and see if the noise goes away. In any event, Fred, it will be easier than pulling the motor and rebuilding it. My Son had a Buick wagon that did the same thing and it turned out to be a loose convertor to flexplate fastener situation. If the noise has been there all this time I think some internal engine problem would have surfaced by now, and taking the load off and having it quiet down sort of leans toward something external in the drivetrain. Don
My pontiac spit the balancer out with the end of the crank bolted to it. Zero symptoms until about 2 seconds before the shit hit the fan, literally. Not that I think that's your problem but yours is easy peasy.
I vote flex plate. If not the flex plate check and see if you have excessive end play in the crank. When you step on it the torque converter will try and come forward (fluid prssure, fins, etc) if your crank is walking it could cause it to make a knocking noise. Rare though. Also check and make sure your torque converter bolts are tight. If they loosened up you will chase a weird knocking noise. Just my $.02. Hope it helps.
If it knocks any at all while idling in park, put your foot on the brake and then put it in drive and load it a little, if the noise diminishes or goes away it is most likely the flexplate.
Sometimes it is the opposite. it will knock a little unloaded at idle. Load it in drive and it will go away, load it in rev and it gets wicked loud!
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