I got on it the other night and heard a pop from the rear end. So I let off went home and saw nothing broke underneath. The next day I got on it again and heard a couple of pops. Still see nothing wrong. Later that night just to see if it would happen again a heard several quick pops. Now I'm worried. It doesn't do it on normal driving and not even driving it hard. Only when I get on it (WOT). A couple of days later I had some new tires and rims put on. 17x9.5 w/ 275/40's on front, and 18x10.5 w/ 295/35's on rear. When the car was jacked and tires were off I noticed the rubber bushing over the axle was wasted. Could this be the noise I'm hearing? Is it bouncing off the axle housing?
Also, now that I have the new tires on when I drive from stop to second I get a pop under the floor board right behind the console. I can feel it hit. Is this a bad u-joint? This is driving me crazy. I got on it today at about 65 mph and heard it again when the car shifted. I felt it hit. But when I get home there is nothing wrong.
Any help would be appreciated. I'm running stock LT1 with hypertech programing and 3.23's in the rear. Automatic Tranny.

how old is your car. it might be anything you said but it might also be your differentail. I heard the same sound on one of my old trucks and when i checked it out, the differentail was dying

Under your car there should be a long piece of metal that runs right next to the drive shaft from the rearend to the back of the tranny. This would definately cause the all of the symptoms you have described.