Guess the horrific noise

[mr_josh]

Here's something I just thought to post because I'm finally actually whittling down the list of issues with my truck.

Background: I bought my truck after someone had already swapped in a mid-'70s 400, car C6, and divorced NP205. They didn't do the best job, but the driveshaft angles seem mostly okay.

The truck had been taken off the road because PO swamped it and hydrolocked (more like, "mud locked") the engine. When I pulled the tranny pan, I was a little freaked out because it had about a quart of water in it, but I ran some fresh ATF through it and put in a new torque converter and despite leaking pretty good from the tail shaft, it actually shifts well. New U-joints all around except in the front drive shaft which has a fair amount of slop in the slip joint, new gears and bearings in both differentials. Transfer case fluid is fresh.

On to the issue: since day one of putting this thing back on the road, it has made a really pronounced grinding sound but only when you're coasting and just barely tipping in to the throttle; the sort of situation where you're driving ~30 mph and you need to feather the throttle a little to maintain a constant speed, the point where you're touching the throttle but the truck is still coasting. If you are coasting without your foot on the throttle or you are accelerating from a stop, it doesn't make the noise. The sound is sort of a hollow grinding / clanging sound, and it sounds like it's coming from the transmission but I can't be entirely sure. The transfer case is pretty much at the back of the cab on this truck, and the sound definitely seems to be down by my right foot, so I'm thinking transmission.

When I first heard the sound I thought, "Well, surely anything making a noise that bad won't last long, I'll just plan on rebuilding whatever fails -transmission or transfer case- when it happens." And I just drove the truck like that. Well, it's been years and miles and it's still making the noise with no indication of impending failure. Any automatic transmission experts care to hazard a guess as to what's up with this thing?

Thanks in advance,

Josh

[stephen44]

just a few quick thoughts

- could you jack the rear wheels up (chock the front wheels and take all safety measures etc) - can you reproduce it - if you can - it should be easy to narrow down where the noise is coming from

when it is making the noise on the road - if you put it in neutral - does the noise go away.

Not saying the answers tell me anything - but might give those who know more info

Good Luck !

[basketcase0302]

Make sure you have someone else there (foot on the brake) when you do it.

[mr_josh]

I'll test that out and get back to you guys, thanks.