I was pushing my car forward in my garage the other day and I noticed it was making a popping noise in the rear as it rolled. It sounds like a u-joint, but they look OK and there is no free play in the driveshaft. It feels like something is binding, I could feel a small vibration through the body every time the noise occurred. I jacked the rear up and spun the wheels by hand and the sound was gone. It only does it with the wheels on the ground. The car is a 95z m6 and drives fine otherwise. Any ideas on what this could be? Thanks.