I have a constant rumble coming from the rear, varying mostly with road speed.
Sounds like a U-joint to me, but we can't find any free play in the shaft and there is no noticable vibration.
Is there a definate way to tell the difference between u-joint grumbling and
an axle bearing problem?

Thanks.

I've seen drivelines that were slightly bent; Perhaps that's the case. However, it would be accompanied by vibration. If it is, try getting the rear wheels off the ground and run the engine in 4th gear. While the driveline's spinning furiously (and I'm sure you know this, but don't get your ponytail or whatever caught in the thing), carefully hold the shaft of a loooooong screwdriver perpendicularly against it and see if it wiggles. If it does, you got yourself a bendy driveline. Even a little bit of wiggle is too much. Has this problem been evident ever since the driveline was put in? A common mistake people make is to put the driveline in incorrectly; It's a little difficult to explain, but the "U" on the rear of the shaft you put in must line up with the same "U" on the piece you're inserting it into. Know what I mean? If someone else can describe this in a more articulate manner, I welcome it. Check to see if that's the case and keep us posted--

Best,

Ben

Signalius et al:
This morning was spent under the car checking some of those things (again).
The driveshaft was checked in v-blocks before it was installed, so, no it is not bent. I also verified the alignment of front and rear sections at the spline. The center support bearing is a new one and looks to be riding in it's mount correctly after 500 miles of use.
I can't say when the noise began. While I've been driving it there have been much louder things happening that would have masked this - like no muffler, no windows, etc.

But I followed advice recieved here and lifted a wheel to spin it by hand.
What I hear is hard for me to interpret, since all the innards of the differential, the drive shaft and transmission output shaft are all involved. It's certainly not smooth & silent like a front wheel would be, instead doing a muffled thump-thump-thump. Same on both sides.

Oh please don't tell me it's looking like wheel bearings.
Gene

I've had wheel bearings go out a few times, and it was foreshadowed by a high-pitched squeaking in one case, and a dry, muffled, continuous wooshing sound (without significant ocillation) in other cars. The squeaking seemed to indicate that I had enough bearings to possibly make it home before it locked up. The wooshing on the other hand, has been with me for years in the 122 and the P1800, respectively, and imagine I'll wait for the death-squeak before replacing them. I am, of course, a stupid, stupid man running the risk of damaged spindles and whatnot, but so it goes. In any event, you said the sound is coming from both sides; It would be unlikely that *both* rear wheels' bearings would fail simultaneously, but it's certainly possible over time. So...

How does the rear-end oil look? Is its level up to the hole? Are there metal shards on the plug? Or perhaps a can of Spam (original flavor) somehow found its way in there?? Perhaps you should take you muffler off and smash out the windows with it?

Gene,

Your response to Phil S. made me think of another possible answer. When you replaced the center support bearing perhaps you did not get the rear drive shaft aligned with the front half. The yoke on the engine side of the center bearing should be in the same position as the yoke on the rear side of the center bearing. Because the rear half of the drive shaft is splined it is easy to install it off a spline or two. This causes the U-Joints to work against each other and can cause the noise you discribed.

Hope this helps,

Ron J

You can probably locate it by getting the rear wheels off the ground and spinning them by hand. A bad bearing will grumble at hand-spun speeds. Failing U-joints vibrate and/or clunk.

Check the bearing that supports the center of the driveline...

Thankyou Phil:
I'll give that a shot, even though I don't want to face replacing those wheel bearings.

I'll look at the center drive shaft bearing too. It's new but I may not have
it installed just right.

Gene

One should be able to diagnose the area of the noise better with the car on a hoist, in a shop. Cheaper than guessing.