'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.