I would say when they get to where you can hear them they are AT the end of their life.

rumble
 

You said "I notice it most a low speed <20 mph and it feels like a rumble that is loudest as you are backed off the throttle and then back on it again. "

This past month I started getting this sound under heavy throttle starts and throttle-off at low speeds.

I was thinking center bearing as well but I'm not getting a lot of vibration at the shifter (which is my quasi-way of detecting driveshaft vibration).

My next thought was to make sure the diff was topped off and not low on fluid.

Please keep us posted!

new symptom
 

I took a ride down one of my favorite local twisties and noticed when accelerating hard through a right hand turn I heard a rumbly whine from the rear, possibly the right side. No sound on hard left handers or normal speed right handers.
Then when coming back up the hill to my house while going over the whoop de doos that form along the right hand turn halfway up the hill
I got the noise again. I also was going faster than usual through that turn and it was the first time that noise has occurred there.
Seems to be when pushing the car harder than normal.
Any thoughts on this development?
I drained and refilled the diff way back when I first got the car.

New tires
 

I put a new set of Michelin Harmony's and bundt wheels on the old girl, crossed my fingers and took a ride.
Oh well, it was a pipe dream to hope that this would cure the driveline noise. Not!
It still vibrates a lot from 35 to 55 and then smoothes out above that
although compared to the flatspotted, bent rimmed, boots that I was running, the car rides much smoother, quieter and sure footed thanks to the new Michies.
I am not sure of my next step since I don't want to unnecessarily throw parts at it. Any more diagnostic help would sure be appreciated.
I am loving driving the car ,vibe or not.

lay off the chile and burritos.

Have you checked to see if the brake calipers are loose? It's a long shot, but it would produce a clunking sound when you let off.

Sometimes the cv joints will wear in the center where they are most of the time and will make noise if you accelerate or let off the gas and make a change of position...this also will happen if you have a load in the back sometimes.

This is not indicative of impending immediate failure but replacement of the half shafts will cure the noise if this is the cause of the noise.

Did you ever figure this out?

Sounds like you narrowed it down to either the rear bearings, axle shafts or the diff.

Reviving an old thread,

My 85 has always done this. It's not a clunk, but a very very low pulsating rumble.

I tried recording it awhile ago, but the sound is so low that I need a better end microphone.

The sound is a rum rum rum rum that is between 20-40 hertz. It's a very low rumble.

I'm nearly positive it is the differential.

i believe that was a conflict in a ren and stimpy episode...

As for the "questionable noise", low rumble describes it well;
in addition to that, it really has a stong rotary vibration that I feel in my seat. As stated earlier, new center carrier and bearing on the DS and the flex discs look and feel good.


The car is parked inside for the winter so hopefully I will get a chance to look things over at some point.

Yup that is the exact same feeling I have. I can definitely feel it in my seat.

Every time I drive I feel it.

Have you had the driveshaft out?
Do you know what an out of balance shaft feels like in the car?
I marked mine before I pulled it and am sure I reassembled it the same way.
I bought the car late in the year and did not drive it much so I have no baseline line for seat vibe but the rumbly noise was apparent from day 1

No I have yet to check that and I have no idea what an out of balance driveshaft feels like :o

Tomorrow I am replacing my axles and I'm going to do a quick check of everything.

However I have the previous owner (stepdad) next to me and he said this very low rumble has always been with the car since he purchased it.

If nothing looks out of place I'm going to leave it be and just let it go. My OCD with this car and my finances can only deal with so much.

Edit: This car is my daily driver. When I got the car in early July this pulsating rumble was the first noise I noticed. Around 60 mph it is pulsating at such a speed that it is constant.

Oddly enough when it is cold I can't hear it at all. I believe it is the differential getting hot at highspeeds.

Sorry to keep this thread coming back but I think I may have pin pointed our rum in the rear.

Behind the differential is a mount that connects it to the unibody frame. When I pulled this off the inside of it was mostly rubber. On my car the mount had most of its rubber hardened.

Fastlane has them listed at around 80$ , but auto haus has them for 40. I'm tempted to buy a new one and see what happens.