So I've had my N/A 1981 280zx for about 6 months now and for the whole time I've had it there has been a sort of thumping sound coming from the rear end. The sounds speeds up with the speed of the car and it gets smoother as the speed increases. At low speeds it almost feels like the car is bumping up and down but after about 45 i can only hear the noise.

Some things that I've had done:
I took it to a shop and they spun both wheels individually and said they couldn't find the noise.
I changed out the differential.
The tires are relatively new and i don't think they are the problem.

Car has about 156000 miles on it
Any help would be appreciated!
Thanks!

 

OK, I'll take a shot since nobody else has yet ... Based on what you say and I'm imagining the thump you hear... It sounds like burned wheel bearings to me. They spin fine with the car up on a rack. But you drop the car, put some major weight on 'em and they have to roll on their flattened faces. Or, their container ring is burnt out for a stretch. So I'll bet on the wheel bearings. Or, it could be a bent axle or drive shaft, something that is turning with the wheels.

Otherwise, I think it has to be one of your tires. This would be easier to check since you can just rotate back to front.

So, my guesses are all in the drive train because the thumps are keeping time with your speed.

I know you will keep going until you solve this - because it will make you crazy if you don't! So let us know what finally works

 

Thank you so much for the input!
Im not sure i have all the knowledge to go after wheel bearing so i may just have to take it in and see if i can get them done.

Maybe i will have them rotate the tires while im there too.

Thanks again!
I'll try to keep you posted on the results!

 

OK. I am only guessing... Better if you take it to a reliable mechanic and show him the thump by taking him for a ride. He may have a better idea.

 

Have you checked the diff mount or u-joints?