My '55 Chrysler's undergoing some much-needed repairs, so I got my old '78 D150 2wd/4spd OD back to use while I fix the car.

It's been sitting undriven since late August, but it started right up after being on the battery charger for a bit. About 10 minutes into the drive back home, though, I noticed a weird noise that wasn't ever there before. It's a kind of howling noise that changes pitch with engine RPMs (less RPMs = lower sound), but it really only happens when the clutch is engaged AND I'm pushing on the gas, even just the slightest amount. At 70mph it sounded almost like a guitar feedback sound or something, and it never really sounds like a metal-on-metal grinding noise (although it could be).

After I got home, about an hour drive or so, I let it cool down, then took it for a spin around town. It only does it noticeably above about 25mph in normal driving, and I did notice that it baaarely makes the noise if the clutch is engaged and I take my foot off the gas. The sound is much quieter and kind of goes on and off when I'm doing that. Still, it's mainly just while driving, and basically if I take my foot off the gas, even if I'm going 60mph, the noise stops.

The only things I can think of are that it's the clutch, or maybe the trans is low on fluid (back when I was driving it regularly, I'd notice a couple drips of pink fluid from the bottom of the trans pan from time to time). Otherwise I'm stumped.

Speaking of being stumped, are factory-type shop manuals available for this truck? I've been driving it for like 15,000 miles and haven't bothered to find one since the truck has been really reliable and it's simple to work on anyway, but it's probably about time to get one...and I really don't like Haynes or Chilton's.

Check your fluids, including the rear end lube. (gear oil)

This type of noise can also come from a "dragging" throw-out bearing. Does the clutch peddle have 1 inch of free play at the top of it's travel? You may want to re-adjust this setting to make it a little wider.

One other test is to pull the fan belts on the wild chance that it is something related to the accessories.
DD

Hopefully it's the fluids, yeah, I'm gonna go out and pick some up. I can tell it's ATF slow-dripping out of the trans pan, but the owner's manual says to use a "DEXRON type automatic transmission fluid" or something. Is regular old dexron III fine to mix in?

I just added almost FOUR QUARTS to the 7 1/2 quart tranny and it didn't seem to make any difference. I guess maybe a bearing got screwed up when it was low, but I'd think it'd at least have helped to add all that fluid.

I noticed when driving it, though, that the noise ISN'T connected to RPMs, it's directly related to vehicle speed. If I slam it into 3rd at 60mph then back into 4th the pitch of the noise doesn't change. While I was under there I did my best to check the driveshaft and as far as I can tell the only "slop" is directly along the rotation axis, maybe 2 or 3 degrees. Aside from that the only other thing I can think of is the differential, and although I don't think it was really leaking, I couldn't even check because there's no filler cap on it that I can find. The only opening I found was a rubber plug on the backside, and unless the fluid level in the housing only goes up about halfway, fluid would've poured out if I removed it. It seems ridiculous that you'd have to remove the whole axle to change the fluid (which I think is a 30,000 mile interval job), but I couldn't see any other way.

The rubber plug half way up the cover is where you add the fluid to the rear axle. It only gets around 1/2 full -- you fill it to the edge of the hole.

If you were 4 quarts low in the tranny, some of the bearings wouldn't have been getting much if any oil. You might have damaged the output shaft bearing, for example. If it's bad it'll make a howling noise, in gear or in neutral, at any rpm, that changes pitch with vehicle speed. Of course, that could also be your differential...

_________________
My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Ah shoot Now that it has fluid, is it safe to drive (just annoying)? Also, does anyone sell a full rebuild kit for the trans? NAPA just lists all the different parts, and hell if I know which type of bearing goes on this particular trans (I think it's listing all 4 speed parts).

Is it likely that the gears got damaged in the trans or rear end, or would it just be the bearings?

Ok, slow down.

First, you found the tranny was low on fluid. Have you checked the rear-end yet? There's no connection between fluid in the tranny and fluid in the rear-end.

Next, you need to find out whether the noise is really coming from the tranny or is it coming from the rear end. You seem confused on that point.

You said there's slop in the drive shaft. There shouldn't be any. Is any of the slop in the U-joints? If so, they're bad. There may be some slop in the spline connection with the output shaft of the transmission or in the differential gears. Make sure that's not what you're seeing -- the end couplings should turn with the driveshaft (remember, no slop in the U-joints).

Have you confirmed it's not the clutch throwout bearing or engine? If it's the tranny or rear-end, Once you're up to speed, you should be able to put it into neutral and still hear the sound.

By the way, you should get a service manual. At least a Chiltons or Haynes manual. that's help a lot.

_________________
My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Yeah, I checked and filled the rear end earlier today. Took 2 quarts of gear oil. That reduced the noise a lot, but it's still there, so some of the bearings must've been screwed up a bit from running nearly dry for 15,000 miles...

The slop in the driveshaft isn't the u-joints, it's like a 2 or 3 degree free rotation if I try and turn the driveshaft along its normal rotation axis. If I just lay under it and push straight up or side to side it doesn't budge.

I doubt it's the engine, unless the flywheel is loose or something, which doesn't seem too likely. Also, if it were the clutch throwout bearing, wouldn't it only make noise when the clutch was disengaged, instead of the opposite?

The clutch bearing floats when the clutch is engaged. It's only actually carrying load when you've got the pedal depressed (Unless it's adjusted wrong -- which is one thing Doug was suggesting)

If it's the engine, you'd have a change with engine rpm, not vehicle speed. As Doug suggested, I just wanted to be certain it wasn't something there.

Sounds like you found it. If you added oil to the rear end (which sounds like it was empty btw); and, the noise got better, that's probably where the noise is coming from. Running either the tranny or the diff without oil is bad...

_________________
My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Oh yeah, I know I never bothered checking it before since it was only a really slow leak (like a couple drops every week, it seemed) from the trans, and I never noticed anything coming from the rear, it's pretty dry. It must have been leaking for a couple decades.

Lesson learned... Take 10-15 minutes to check all your fluids every time you change your oil. What else have you got to do while waiting for it to drain?

You'll probably be OK for a while; but, you're eventually looking at a rebuild of the rear end -- bearings, shims, seals, etc. Also have to look at the gear surfaces and measure gear lash. They might be worn out of tolerance or scored up.

_________________
My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!

Does anyone sell a rebuild kit or do I have to figure out what parts I need on my own? Same thing goes for the trans also.

Just about anyone can get you the rear end bearings and seals. If you need gears or a full diff then you can check with Summit or Jegs or call one of the gear vendors direct.

Transmission parts are harder to find. Napa and Advance have some of the parts. I found these on the web & they claim to have rebuild kits:

Drivetrain Specialist Inc.

National Drivetrain

Motive Gear

And these guys cater to Mopar people and specialize in A833's:
Passon Performance

Having said all that, I would strongly recommend you get service manuals and do some reading before attempting either. There's a good article on the main .org site on the transmission. Assembling a rear-end or a tranny is a little bit art, requiring some skill to get the gears to mesh properly (lots of adjusting, shimming, assembly, measurement, dissassembly & try again....)

_________________
My truck is a Frankenstein creation
Built from the dead carcasses of others
And brought BACK TO LIFE!