Ideas for diagnosing random terrible noise?

[Rollo Grande]

I realize this is pretty vague, but maybe you've had a similar problem and have some ideas.....thanks in advance for any leads!

I have a new-to-me 1970 C20 350 with the the 4-speed granny trans. G80 rear axle. I don't use the granny so 2nd is first, 3rd is second, etc.

There is an occasional metallic bang/jerk that happens upon acceleration. I am only driving around town at pretty low speeds, 20-40 max, so it's occurring in 2nd gear, sometimes in 3rd, and not in 4th that I can recall.

Felt to me like it was in the driveline, I worried it was the diff going bad, or something in the transmission. Yikes $$$$.

But today had a local crusty old transmission guy (the best kind!) test-drive the truck and when we felt it go BANG he said, "I don't know what that is, but rear ends are pretty consistent when they go bad, and your trans sounds normal for an old truck. I'm pretty sure it's not in your driveline, sounds like it's coming from up front."

So I guess that's good, or at least less-expensive news, but not sure where to look.

Some wild guesses: Motor mount? Frozen wheel bearing? Seized brake part? (The brakes only recently were restored to power assist but they seem to work fine, and the noise was there even when driving with Flintstones brakes.)

What would you investigate next?

[maverickmk]

Bad brakes or bearings will cause the vehicle to pull to the side that is causing the issue. Have you looked at the motor mounts? They usually will make a sound when taking off - the motor will lift, then when letting off the gas it will drop the motor down making a noise. Also, a bad driveshaft U joint will do basically the same thing. If you hit the gas and let off and hit the gas again and let off each time you should hear a clunking sound.

[Barnfind46]

I think Maverick is on the right track. If the truck was an automatic it would not be as apparent of a problem but with the stick trucks you would notice the mount more. I had an old truck that made a really annoying sound from the passenger side. After I divorced her the noise went away.

[HO455]

Does the truck have a limited slip (posi) rear end? They can make some pretty unsettling sounds (as well as making the vehicle jump) when they go bad.

[Sheepdip]

Start at the front and work your way back, motor mounts, trans/bellhousing mounts, driveline carrier bearing and u-joints. I'll bet you'll find one of those is the mystery noise.

Get a buddy to help... out of harms way of course, set the emergency brake and put a strain on the drive train feathering the clutch, if its any of those you should be able to see it move.

['68OrangeSunshine]

Bad differentials go ''clunk'' on acceleration, before they go ''BANG'' and quit.

[Keith Seymore]

Originally Posted by Barnfind46

I think Maverick is on the right track. If the truck was an automatic it would not be as apparent of a problem but with the stick trucks you would notice the mount more. I had an old truck that made a really annoying sound from the passenger side. After I divorced her the noise went away.

This particular noise: was it consistent, or intermittent?

Asking for a friend.

K

[rpmerf]

Originally Posted by HO455

Does the truck have a limited slip (posi) rear end? They can make some pretty unsettling sounds (as well as making the vehicle jump) when they go bad.

G80 = Gov Lock

[rpmerf]

Since this truck is new to you, I would start with going front to back over the truck, so you know what you got. Brakes, wheel bearings, ball joints, tie rod ends, pitman arm, idler arm, control arm shafts, motor mounts, trans mount, u-joints, carrier bearing, diff, track arm bushings, trailing arm bushings. Grease up what you can while you look around.

With driveline noises, I've jacked up my truck and put it in drive. I parked my suburban in front of it with the bumpers touching, just in case. Helped me find a bad carrier bearing.

Figuring out how to recreate the issue will help figure out how to resolve it.

[A1971Blazer]

If it has a limited slip....drive it in a tight circle...if it's the posi unit...you'll know really quick...

[Rollo Grande]

Ahhh these suggestions are setting up a good project plan, thank you! What a helpful forum.

A mechanic buddy is letting me use his shop, so I come over in the evenings and flounder around, and if I get stuck he bails me out. Pulling in last night he yanked back and forth on the bed and said that it might be loose and shifting on the frame. I don't think that's the source, but it did look a bit loose compared to the cab. He did that also while I was under the truck with a flashlight, I couldn't see anything wrong, but maybe not looking at the right supports.

I'll be solo for the next few weeks while he's on vacation so limited to noob wrenching activities and probably a lot of cleanup and prep. I am good at "paint and shutters"....