I have a 2000 v6 mustang and I replaced the u-joints recently, when I put drive shaft and tried to move car it started making a popping sound, the faster I drive the faster it pops, however when the rear tires are off the ground there is no noise, what could it be?

I would raise the car up, and while someone rotates rear tire forwards and back, watch U Joints carefully for free play.

Just replaced u-joints... not it

I have seen newly installed U Joints where a Pin got out of Place.
How did raising the car and having someone move tire back and forth work?

Never mind, sold car

No more Problem. Good Deal.