Loud clunk after lifting off accelerator

[n33k0]

Hello,

My 1972 C10 Stepside (350/TH350) has no shortage of annoying clunks and squeaks. The noise that is next on the to-do list is a loud clunk that only occurs when I lift my foot off the gas. Sometimes it only clunks once, sometimes two or three times in a row, but only when I lift my foot off the accelerator pedal. I have found that I can avoid it by gently lifting my foot off the pedal, but if I have to jump from the gas to the brake (getting cut off in traffic), something under the truck will most certainly clunk loudly. It only happens when the truck is in gear, as well; pressing the accelerator then quickly releasing it while in neutral does not cause any clunking noise. Any ideas what would cause this?

[BRL]

First thing I would do is crawl underneath and attempt to rock the driveshaft in both directions to see if the U-joints are shot or the differential has excessive slop.

[Wyatts Classics]

Originally Posted by n33k0

Hello,

My 1972 C10 Stepside (350/TH350) has no shortage of annoying clunks and squeaks. The noise that is next on the to-do list is a loud clunk that only occurs when I lift my foot off the gas. Sometimes it only clunks once, sometimes two or three times in a row, but only when I lift my foot off the accelerator pedal. I have found that I can avoid it by gently lifting my foot off the pedal, but if I have to jump from the gas to the brake (getting cut off in traffic), something under the truck will most certainly clunk loudly. It only happens when the truck is in gear, as well; pressing the accelerator then quickly releasing it while in neutral does not cause any clunking noise. Any ideas what would cause this?

It may be one of the U joints on driveshaft. Crawl underneath and try wiggling the shaft. Make sure the tires are blocked from rolling. Try putting in neutral and hand turn the rear shaft to differential. If it has a clunking sound and is loose you will see and feel it.

[n33k0]

Thanks for the replies. I had a suspicion that it was the drive shaft, but I don't know enough about transmissions to verify the problem (wiggling driveshaft? I guess it's supposed to do that )

I'll crawl under the truck after work, check it out, and report back.

Thanks again, fellas.

[Mike C]

Does it do it when you have foot on brake and shift from park to drive and then drive to reverse? That sounds like a u-joint. Really whacked carrier bearing could clunk. But a ripped motor mount is also high on list if it only happens when you back off the gas. Especially on a 71-72 truck with throttle cable. On an earlier truck, the mechanical linkage sticks at WOT when motor mount breaks...

[n33k0]

Mike C,

I haven't noticed any unusual noise when shifting into or out of gear... certainly not the same clunk that I described earlier. Sometimes I'll feel a light thump if I'm reversing and switch to Drive before the truck completely stops rolling backwards, but it's not loud or strong enough to make me think something is wrong or broken.

[outdoorsman310]

It could also be a trailing arm bushing, loose cross member, or broken mount.
what if you press the gas and let off in 1st as if you are trying to get the truck to bounce up and down and do a wheelie?

[Mike C]

Get in the truck and start it. Drop it in gear. Press on brake with left foot as hard as you can. Bring engine revs up to 2000-2500 and then step off accelerator. What happens? If you can get help, have somebody else do this while you look at the motor to see if it's lifting up on the driver's side.

Please stand to the SIDE of the truck in case your helper's foot slips off the brake...

[chip46wis]

Check the U-bolts holding the A arms also they do crack and rust.

[davepl]

Originally Posted by Mike C

Get in the truck and start it. Drop it in gear. Press on brake with left foot as hard as you can. Bring engine revs up to 2000-2500 and then step off accelerator. What happens? If you can get help, have somebody else do this while you look at the motor to see if it's lifting up on the driver's side.

Please stand to the SIDE of the truck in case your helper's foot slips off the brake...

Uh... do not do this with solid linkage. It might be OK with a throttle cable, but with rod linkage consider this:

- You hold the truck with the brake
- You bring up a full converter load on the engine at about 2500
- Driver side mount was broken or breaks
- Engine torques up as it rotates
- That pulls on linkage
- Now you've got "cruise control" or "crazy high idle" in your driveway.

At that point you could either (a) panic and throw it into neutral and hope it doesn't overrev before the engine physically falls back down, or (b) turn the engine off and hope it doesn't diesel and detonate really bad because it's still got the throttle open.

GM engines of the late 60s were notorious for breaking mounts (bad mounts, not huge power), and the 69 and earlier cars with solid throttle linkage caused some serious events. We've all seen the "aircraft" cable loops to tie the engine to the suspension, a cheap bandaid for the problem that GM provided.

I think your suspicion that it might be a mount is spot on - that's my guess too. Just be safe checking it.

When I was a young man decades ago I'd check it myself by having the hood open, doing exactly what you say, while looking through the windshield and the gap between the cowl and the open hood to see what would happen. So I speak from experience - some exciting experiences!

[n33k0]

I gave the drive shaft a good shake, but it didn't budge. Pushing and pulling front to back, left to right, at the transmission and the diff, no movement or noise. I did hear what sounded like the bed rocking back and forth, so that's something I'll have to do when it's not raining outside. I don't know if a loose bed bolt is the cause of the clunk, but it's probably contributing a lot to the noise the truck makes going over bumps.

I had never considered the motor mounts, it always felt like the noise was coming from below or behind me. Is there any way to check the motor mounts without the engine running? I'll also double check the u bolts, even though they seemed ok when I was doing suspension work on the rear a few months ago.

I appreciate the help guys. I don't have much free time to work on the truck, so these ideas give me a good game plan to follow when I finally get a chance to get my hands greasy.

[tcrist]

You can always put a floor jack under the lowest part of the oil pan (with a 2x4 or something on the pad of the floor jack) and raise it slowly and see if the motor raises on one side or the other. Don't crush the pan though. Might be the passenger side motor mount lifting and letting something move and touch something else underneath. Like maybe the exhaust touching the frame or somewhat.

[Mike C]

With your throttle cable it will be a non-issue to do with the engine running. You have been driving the truck it sounds. If it was a mechanical linkage and a broken drivers side mount you would have known it awhile back...

[hooverfish]

Originally Posted by n33k0

Mike C,

I haven't noticed any unusual noise when shifting into or out of gear... certainly not the same clunk that I described earlier. Sometimes I'll feel a light thump if I'm reversing and switch to Drive before the truck completely stops rolling backwards, but it's not loud or strong enough to make me think something is wrong or broken.

Rolling backwards and putting into drive before you stop rolling .... this will ruin your universal joints over time, if in fact they are not already gone.

Best to come to complete stop before changing direction. This is a good practice with any automatic transmission.

[n33k0]

Quick update:

I haven't had a chance to check the engine mounts, the only time I find to troubleshoot the truck is weeknights after work and I don't want to be "that guy" in the neighborhood revving a loud engine when the neighbors are sleeping.

While doing a visual inspection of all the mounts I noticed the header on the driver side was hitting the frame and the front suspension (lower control arm) I think that is the source of the noise. Maybe the engine is moving just enough to cause the clearance issue and resulting noise?

Add: I found this pic that shows a similar issue on another truck, complete with dent in the tube. Maybe that's just the way things are with these headers?

[Freeze]

If that is the case, the clunk is most likely not from the engine moving, but from the suspension moving. Are you accelerating hard enough to pitch the truck back? accelerating hard and then lifting can cause the front of the truck to raise and then lower back to 'static' position. Of course with 40 year old springs, the truck is probably dropping more than it was designed to and then the lower control arm is bumping the header. My .02