Technical Help! Sbc "zap" noise only at speeds.

Hello.i have a weird noise coming from my sbc in my coe.
It sounds like an electrical zap noise and it's random. Only at speeds and around 215-220 degrees, sometimes it dissappears.
I had a mechanical temp gauge and the noise would seemed to be in time with the temp gauge as it fluctuated or jumped from 210to 220.
I thought it was the wind and a loose wire but it makes the noise when I hit the brakes and decelerate then the noise goes away.
Centerbolt engine,non mechanical fuel pump,and not sure how many miles.
It runs strong doesn't smoke,leaks a bit of oil from the valve covers very tiny amount.
Sounds like a loose wire hitting the floor of the cab..I checked every wire,loose bolt,etc.
"Zap....zap..zap-zap...zap...zap...zap."
Any ideas?
Thanks in advance.

 

Any chance its HEI ignition? I have seen them burn up the metal connecting pieces,and still have enough power to jump across,and run. If it is take the coil/top parts off,and check condition.

 

Is it one of the LT style small blocks ? I had one years ago with the optispark..? distributor that sits under the water pump. The housing developed a crack and it would arc over to the pump visibly and still run too. It made a sound like youve described.

 

Does your voltmeter needle jump when this happens? Could it be something that sounds electrical, but is something else?

 

Maybe a small coolant leak under pressure making the sound?

 

Zap like blowing through a blade of grass? Or more like a tap?

 

You need louder pipes!

 

Or like electrical arcing?

 

Check grounds. Look around base of temp sender, if elec. Do you have a suitable engine-to-chassis ground?

 

it's not a lt block.

Don't have a voltmeter.

more like a tap.

I do.

it's a mechanical temp and yes good grounds all around

 

Do you have a car stereo in there? If so pull the fuse and eliminate it from the list of possibilities

 

Maybe the temp sender line is rattling on something.

 

Alternator over-charging, arcing?

 

You mentioned that the temperature gauge would jump about the same time. Could the noise be a water hammer? A buddy has a 400 SBC and his engine temperature would jump around along with a water hammer sound like old plumbing in a house. He finally fixed it by running coolant return lines to the rear of the intake. Don't know if that's what you have but the temp swing and noise is exactly what he had.

 

Maybe a sticky thermostat? I had an OT Chevy truck that made weird tapping/zap type noises. It was the thermostat that was sticking...or stuck. Might have caused that rig to run hotter than it should and create steam. That might be why the noise coincides with a spike in temp.

 

Are you sure it’s a zap , zap and not a phfffft or a ratatattat or grrrrrRRRRrrrr or a whap whap ta da ? Ok jokes aside , you have ruled out a all mounts , chassis , drivetrain, suspension? When I worked for the power company, a guy came in and said he had a wrap wap sound, increase with speed and decreased when he slowed down . Threw on the lift and he had a branch wrapped around the driveshaft. Good times

 

It probably is a engine related system and you are going to have to put ear to and play with the throttle. No bearing noises from alternator etc. ? Water pump or a belt tensioner ? Maybe get a piece of tubing or a stethoscope.

 

Lastly maybe you need a second set of eyes , ears , hands . Wish I was near enough to help.

 

And you say it runs strong so nothing ignition related like cracked wire(s) easy to spot a night cracked cap? Is it a basic Chevy with points/HEI , alternator maybe power steering, v belt or surpentine set up ? A/C or not ? Sorry for the run on but just want to clarify every component you have . No leaks like vaccum etc Last edit hope it’s not maybe more than one thing .

 

Kind of
"Zzzzzz... Zzzzzz... Zzzzzz..."
or maybe more like
"Tinkity tinkity tinkity..."

 

Somebody want a road trip to Vegas , baby ?

 

I wonder what a plane ticket costs from here. It's a commuter flight, if you can believe that!

 

Bring it up to zap speed, knock it into neutral and let the revs die off. If still zapping it's something other than engine. If it goes away give it some revs while still in neutral. If the zapping comes back, engine is the culprit. If it persists, see a doctor.

 

A busted component in the suspension can bind and then let go with a cracking sound. I had a broken leaf spring which took me a while to find, which would do this.

 

Thanks for all the suggestions.
I forgot to mention that I replaced the faulty temp gauge to a vintage SW mech gauge..still have the noise.
I crawled under the truck yesterday and found a cab mount bolt loose..I hope it's the culprit.

I did turn the engine off one day while it was making the noise and it didn't stop.

It makes it hard because I'm sitting over the engine and I have to take the seat out to remove the doghouse otherwise I just raise the doghouse and crawl under my seat,lift the doghouse 4-5 inches and work through that slot.

I'm hoping that the noise goes away with the cab mount bolt that I tightened yesterday.

 

While you're under there, get a spray can of silicone lube and give the body mounts, and any other rubber bushings, a generous coating.

 

Update.
I added half a gallon of water to the radiator before I drove home from school and once the engine got hot and I hit the freeway, the noise wasn't random anymore,it was more pronounced and more often.
I'm guessing whoever said to check the thermostat is right..
I'll take it out and replace it.
Maybe flush my entire system.
Thanks

 

Narrowing it down. Good deal.

 

While you're at it, check that your radiator isn't clogged up. Fill the radiator, and with the cap off, remove the bottom hose. Your radiator should empty in like 3 seconds. If it doesn't, take it out and turn it upside down and back flush it.

 

Again, guys are using those heat-sensing guns to quickly find what's hot and what's not and it can help to find problems with coolant flow.