'67 122s, converting from a BW35 to a M40. Picked up a donor driveshaft, installed a new center bearing and rubber, and dropped it in. It was making a scraping/whirring noise so I thought it was the u-joints, and I dropped it off at a driveshaft specialist. They replaced all three u-joints, balanced, and even painted and pre-lubed it. Really nice work. I was so excited that I rushed to put it in, bolt in the seats and take it for a ride. I also disconnected the speedo cable.

I pull out of the garage onto the street ... and the noise is STILL there. Now it mostly happens on deceleration at low speeds. Sort of a whirring, scraping noise. It's fairly loud. It doesn't really happen when the car is accelerating, cruising, or decelerating until you get below 5 mph, after going faster than say 20mph. It is still pegged to actual speed, not engine speed.

I have two theories. I had put a new IPD center bearing on the car. It didn't spin super freely but I figured that it would "break in" or something. Now I'm suspecting that the bearing I got was bad out of the box. I did also get a new rubber donut from IPD, but I didn't think it needed to be greased inside to carry the bearing. I could have been wrong on that point too. So I may have a potentially bad bearing running dry in the rubber donut ... not sure, and the manual's no help in this regard.

I think I'm going to try spraying some Tri-Flow on the bearing and donut area, just to see what happens. I think the bearing is sealed so it might not do anything, but if it quiets the car down I'll know that it's something in that area that's not kosher.

The other thing it could be is the rear diff. Others have suggested this. I'm going to pick up some 80-90W GL-4 (the manual says straight SAE 80 but I doubt anyone carries that) and replace the fluid. I've never touched it, and it couldn't hurt to change it out.

Anyone have any other ideas of what I can check? Am I going down the right path here? I am extremely frustrated ... this driveshaft has been taken in and out at least six times now already.

When I swapped a manual into an automatic body, I found that the output flange(s) to front section of driveshaft would intermittently make contact with the cantilevered rear trans support bracket. I even replaced the mount. I believe that I ended up shimming the mount in order to get rid of the contact. Check the flange for shiny wear spots (and with a mirror) the trans mount support bracket.

There are two different mount brackets. The correct one for either M40 or M41 in an Amazon is the taller one, and can easily be identified by having an angle on one side. The shorter, incorrect one is rectangular, without the angle.

Bearings are not to be "broken in". They only worsen in condition as time goes by.

Can you jack the car up, put it on stands, in neutral, go lie underneath it, and spin the driveshaft by hand? According to your description you should be able to hear where the noise is coming from, if it's being generated aft of the gearbox input shaft.

I think I fixed it ... I sprayed some Tri-Flo liberally around and into (I think) the rubber donut area. Now there's absolutely no noise. I'll keep the bottle in the car in case it comes back. At some point in the future when I'm under the car I might take off the driveshaft, remove the rubber donut, and properly lube it with some moly grease. But for now I think it's good.

I think I'll swap out the diff fluid anyways. Trying to just work through the entire car and hit all the "last time changed unknown" fluids. I'm sure it's old and needs replacing anyhow ... and it's the last fluid-holding part of the car that I haven't flushed and replaced.

Replacing unknown fluids is always a good idea. I like do this just to get up to speed, except with anti freeze, if it checks out as good. I also like to add the magnetic drain plugs where applicable. I recently changed the auto trans oil in my 2003 Chevy pick-up. Not a job for the weak of heart, by the way. Worse than replacing the rear brake dust covers. Anyway the magnet that sits in the bottom of trans oil pan had a beard of ferrous fuzz, not filings, on it. Not that bad, I thought for a vehicle with 70K on the od, but a barometer of what was going on in there. Everything else looked okay to me, but what do I know?

Why does a rubber donut make a scraping noise?

Not long ago I replaced a split rubber mount. I'd bought a new bearing at the same time. When I checked the old one it seemed pretty smooth and spun round freely. However when compared to the new one it was obvious that it was worn. The new one didn't spin much as it's a sealed bearing and the grease doesn't allow it to. Much smoother though. You can't grease it from outside so I can only guess that the bearing wasn't seated properly in the mount and your extra lube has settled it in properly.
Your description of the noise does fit a rusty or worn out pilot bearing but that's not easy to check. If it does it when reversing slowly up a slope I'd be more suspicious of that possibility.

That's an idea, but I did replace the pilot and throwout bearings when I installed the transmission (as well as a new IPD Sachs clutch kit). I'm pretty sure the pilot bearing is in there properly, as I did secure it with the correct Volvo "circlip" type thing that keeps it in position.

I've never heard the sound in reverse, up mild slopes, but I'll keep an ear out for that possibility.