I have a vibration that I feel in my lower back sometimes when accelerating that I'm not sure where it's coming from. It's kind of intermittent, but everyone that has driven the car has felt it at different times.

I have owned the car since 2001 and it's been parked for a couple of years because of a RR wheel bearing that needed to be replaced.

I don't remember the vibration being there before, but I have changed a bunch of stuff and maybe it's always been there, but all the other stuff being loose hid it before. As I said it doesn’t happen all the time, but it’s most noticeable under acceleration.

The car tracks straight and is fairly tight, for a forty + year old C3.

Things that have been changed are;

Rebuilt trailing arms w/bushings
New strut rods
New shocks
New tires
New brakes, rotors, calipers, master cylinder soft lines
New front wheel bearings
New front A-arm bushings and ball joints, tie rod ends
New transmission mount (it was vibrating before and I thought that might be the problem, I was wrong)
Four wheel alignment
New exhaust
New rag-joint
The Differential bushing looks good, but I have no idea of its condition or if it's original.
The half shafts are original, u joints felt good, but they were not replaced

 

Could be a wheel/tire...drive shaft...1/2 shaft...my 86 had a weird odd engine resonance that you could "feel" in your body...reason 101 I dont care for c4's