grinding/scraping noise when I let off the gas

[alden]

I know I've got another thread going about another issue right now but it seems if I post additional questions in previous threads then usually get missed so hopefully no one is offended by my second thread this weekend. I've only taken the truck (1959 Fleetside, stock 235, 4 speed) up and down my street so far just trying to figure out all the little things needed to get it truly roadworthy and the latest is a pretty bad series of noises coming below the cab when I let off the gas.

When I first took off today it made a bit of a clanging/grinding noise but that went away once I got up to speed (as in 20-25 mph), then when I let off the gas there was a pretty loud grinding/whirring noise until I got down to a near stop. I was thinking this sounded like a universal joint issue but I'm really quite ignorant on all of this so I could be just talking out of my bum. Any thoughts on this?

Thanks!
Alden

[Bishops Trucks]

Could be a wheel bearing. It is definately something in rotation.

[alden]

Originally Posted by Bishops Trucks

Could be a wheel bearing. It is definately something in rotation.

Is a wheel bearing something I would be able to diagnose by just jacking up each wheel and spin them by hand or does it need to be under load?

[Steve@OldSub.com]

That it happens when you let off the gas makes me suspect general drivetrain issues. It could be a rear bearing, but it could also be in the transmission or the universal joints or the hanger bearing if it has one.

I'd start by crawling under the truck and shaking things. If its that noisy it might very well move enough to make clear what is failing.

If that doesn't do it, carefully jack it up and block it so it can fall. Then crawl under and turn the drivetrain while listening for the source of the noise.

Since it happens when you let off the gas its something that is in compression and quieted when the power is applied. That should be the first clue to what it is.

[alden]

Originally Posted by Steve@OldSub.com

That it happens when you let off the gas makes me suspect general drivetrain issues. It could be a rear bearing, but it could also be in the transmission or the universal joints or the hanger bearing if it has one.

I'd start by crawling under the truck and shaking things. If its that noisy it might very well move enough to make clear what is failing.

If that doesn't do it, carefully jack it up and block it so it can fall. Then crawl under and turn the drivetrain while listening for the source of the noise.

Since it happens when you let off the gas its something that is in compression and quieted when the power is applied. That should be the first clue to what it is.

I went under the truck and started shaking/turning the driveshaft in gear and in neutral. In neutral there is about 1/20th (said 1/4 originally, edited for accuracy) turn of play before the wheels being on the ground stops it, in gear there is maybe 1/100th of a turn of play (but it can be shifted slightly either direction so it's not at zero play). I put the rear end on jack stands and ran the truck in forward and reverse and didn't get any noise at all (braking didn't induce the noise either). I also turned both rear wheels by hand with the truck in neutral and only heard a slight scraping noise in the driver's side rear which I am thinking may be a sticking brake shoe.

Further thoughts? I can take video of anything that might help diagnose it if that would be useful.

Thanks!

[Steve@OldSub.com]

I think a 1/4 turn is too much play in the rear. I'm not a differential expert but I'm guessing you have a rear axle problem.

That its tight when in gear suggest the transmission isn't the problem. Did you try turning the driveshaft from the rear while it was in gear? If its tight that would suggest the universals and hanger bearing are all okay.

Since you are seeing a 1/4 turn at the rear you should work to be sure exactly what is turning that far. Could it be the universal joint? Could the pinion nut be loose?

If the pinion shaft itself can turn a 1/4 with the wheels on the ground I'm going to say you need to rebuild or replace your rear.

[alden]

Originally Posted by Steve@OldSub.com

I think a 1/4 turn is too much play in the rear. I'm not a differential expert but I'm guessing you have a rear axle problem.

That its tight when in gear suggest the transmission isn't the problem. Did you try turning the driveshaft from the rear while it was in gear? If its tight that would suggest the universals and hanger bearing are all okay.

Since you are seeing a 1/4 turn at the rear you should work to be sure exactly what is turning that far. Could it be the universal joint? Could the pinion nut be loose?

If the pinion shaft itself can turn a 1/4 with the wheels on the ground I'm going to say you need to rebuild or replace your rear.

I just checked again and I was way off. It's maybe 1/20th of a turn in neutral, still closer to 1/100th of a turn in gear.

I can't see any play between the universal joints when I turn either side in opposite directions. The only play I can notice, and I may just be seeing things, is at the driveshaft center support, but I didn't think that was an actual joint so I don't know why there would be play there.

[Steve@OldSub.com]

Originally Posted by alden

I just checked again and I was way off. It's maybe 1/20th of a turn in neutral, still closer to 1/100th of a turn in gear.

That's much less scary. Your rear is probably fine if aged a bit...

Check that support bearing carefully. Its just the kind of thing that would grind and scrape when letting off the power and yes it does have a bearing in it. In my experience its the cushion that holds it together that fails and causes the problems, but who knows what may be giving yours trouble.

They aren't real expensive nor hard to change. I've done it laying in the street at midnight. I might have been 25 years younger at the time...

[alden]

Originally Posted by Steve@OldSub.com

That's much less scary. Your rear is probably fine if aged a bit...

Check that support bearing carefully. Its just the kind of thing that would grind and scrape when letting off the power and yes it does have a bearing in it. In my experience its the cushion that holds it together that fails and causes the problems, but who knows what may be giving yours trouble.

They aren't real expensive nor hard to change. I've done it laying in the street at midnight. I might have been 25 years younger at the time...

ok, well that's a pretty big relief then Autozone has them for $20 so I will pick one up later today.

completely off topic, but I can see where to drain the transmission fluid, but where do I fill it? Do I need to pull the shifter to fill it?

[dwcsr]

If it almost goes away in 4th but is more pronounced in lower and slower gears I would suspect the countershaft in the trans. Countershaft would also make a rattle noise in neutral while idling and clutch out. then stop in gear clutch in.

If its less noisy under power and louder letting off then it maybe the rear. I would drop the drive shaft and then check the play. The trans in any gear but 4th see what kind of play you have when you turn the yoke. do the same for the rear when the shaft is not connected. push up and down on the yoke and see if there is play.

What ever is making the noise is grinding metal so it will show up in the oil as glitter and silvery particals like metal flake kind of appeance. Hold a magnet in the oil stream when you drain it. If you get a lot of fur on the magnet thats the component thats bad.

[Steve@OldSub.com]

Have you rattled that thing enough to be sure its the problem? Also I'd take it off and carry it in with me to be sure you come back out with one that will fit. I'm not impressed by the counter help at those places these days.

The SM420 has both a drain plug and a fill plug. This diagram should help find them. SM420 Exploded View

[alden]

I picked up a new center support bearing today, I'm a little confused though, do I just undo the bolts holding the old ones, let the driveshaft drop, and that will give me enough room to replace it, or does it need to be disconnected somewhere else as well?

Thanks!
-Alden