[tooski]

I know it's basically a crapshoot, but here goes.

My drivetrain has developed a tapping.

It is absent until the trans is starting to show temperature. Steady cruise it's difficult to hear. As throttle is applied it becomes more evident. The heavier the throttle, the louder the tap. Put it in neutral and rev the engine - no tapping.
I took a couple of vids with the rear wheels off the ground.
First vid I put in gear (r) at the 25 sec mark. Seems to become faint at 30 sec, a bit difficult to hear as the revs go higher.


https://youtu.be/dOXdykAKpsQ



The next vid I unbolted and tied back the converter. Engine only, no tapping.


https://youtu.be/BFFGsS1P2oc



There is no problem running the trans through the gears, but only did that once coming back from cruising.


A common diagnosis is loose converter and/or flexplate bolts. No on this one.


I have very little experience in tranny work but I am willing if I could get a clue as to what could cause this with the trans still functional.
At any rate, the trans is coming out.

[Scott65]

Parking pawl loose, or broken spring? This only requires pan removal to check I think.
ETA: may require valve body removal to fix...

[Schurkey]

Chipped gear tooth? I had a Muncie M20 that did that. Got louder and louder. I was in high-school. I turned the radio up. One day, I'm a block away from home, and the rear wheels lock-up and the engine stalls. Caught a chunk of broken tooth between some other gearteeth.

No reason a planetary gearset, or a ring 'n' pinion couldn't make noise like that. Or a transfer case gearset.

Exhaust leak? Worse under load and fully-warm, silent when there's little load or cold. Unbolt the converter, the engine no longer is under load.

[chuckies76ta]

Just a thought, Check your balancer bolts. Happened to me and a friend of mine.

[25stevem]

Your flexplate may be cracked.

[tooski]

I have also come across the exhaust leaks, convertor bolts, flexplate cracked, flexplate bolts.
I'll have to set up a camera in various spots to try to pinpoint the noise.
Thanks much for the responses.

[chiphead]

I'm hearing a lot of sheetmetal tapping type noise, mixed in with some exhaust and something rattling/crunchy. Exhaust hitting something, exhaust leak, flexplate? May help to relocate camera, as I'm hearing a lot of different metallic sounds in the background.

[Sirrotica]

Likely a crack in the flex plate, the clue is it goes away when you disconnect the convertor. That dash looks to be a chevy truck, and they crack flex plates pretty regularly.

If the convertor lugs aren't flat within say .020 the sheet metal in the center bends every engine revolution, eventually they fatigue, and snap the sheet metal, and then the ends move over each other as the flex plate rotates. They can make noises that mimic a rod knock, or sometimes just a slight click, each one seems to have it's own distinct noise.

Every cracked flex plate I've had to diagnose, usually has tell tale rust, and fine metal dust near the crack visible when the convertor is pushed rearward. That's typically what I look for when making a diagnosis. There is no set pattern where they crack, as the misalignment can be due to different circumstances.

If you determine that the plate is cracked, there is no need to completely pull the transmission out of the vehicle. I have two 3/8 inch threaded rod that I use for pilots that screw into the back of the block, and just support the transmission, slide it rearward enough to use an box wrench to loosen, and tighten the crank to flex plate bolts, slide the plate out, and slide the new one in, typically a 45 minute job. Saves draining the transmission fluid, removing the driveshaft, disconnecting all the linkages, and cooler lines. Flat rate mechanics usually do it in this manner to save time.

I hope this is of value to you.

[tooski]

The car has all sorts of rattles and squeaks (the solid engine mounts probably have a bit to do with that). The tapping is faint in the vids. It is a bit more noticeable during a road test.
All info is of value to me.
The car is a Chevy II with a Pontiac Engine. The flexplate is a PRW SFI unit, not the stock sheet metal type. Arp flexplate bolts torqued with thread locker. 400 miles on it. I should have given this info to begin with. When I pull the converter up to it all the converter lugs are tight against it. I could not see any of the visible signs mentioned. I plan on having a better look and check the flexplate for runout.

[Half-Inch Stud]

Note: only pull converter about 3/16" forward. May need to space the converter with washers to keep front pump engaged.

[tooski]

Just updating this as some dislike not hearing about problem resolutions, or in this case not.. I called the tranny builder who checked the rebuild and did an upgrade. He said there is nothing inside the trans that could cause a heavy metallic tapping. He also said it's the flexplate.

I changed the flexplate and it made no change.

Then I used a microphone hooked up to my cellphone and recorded the noise at several spots on the underbody. The most pronounced was on the right side under the frame, even with back of the engine. Really sounded like someone was tapping heavily on the exhaust pipe or the oil pan.

I don't know how or if I can upload a sound clip.

On a side note, just for clutching at straws, I disconnected my vacuum advance. Noise was still there, just not as loud. I have a difficult time believing it could be detonation as I have had detonation a few times with it always seeming to come from inside the top end of the engine.

I have also heard rod knock, but also sounds internal to the engine. This one sounds external.

At any rate, after spending too much time to decide on a new flexplate, my time this spring in AZ is up.

I'll be back at it again in the fall.

Thanks to all for the responses and support.

[Half-Inch Stud]

Is engine oil pan laying on the front Crossmember?
Is exhaust manifold/ header contacting front Crossmember?

Rod bearing getting ready to spin?