General LSX Automobile Discussion - strange thumping when under power.. need help

the vehicle is a 00 WS6 with a t-56. after market mods include a PST carbon fiber drive shaft and a moser 12-bolt

The issue I am having started today. I went to switch gears and when i went into 3rd I got a loud thumping. It was almost like the tire got a huge nail in it. As i went faster it got louder and quicker and vise versa. The strange part is when I push the clutch in or take it out of gear, the noise stops. I jacked up the car and ran it, and found the noise to be coming from the drive shaft (or so it sounded like). I disconnected the drive shaft and ran my car though the gears, no noise. I spun the rear wheels manualy, no noise. I even drained the rear gears and found clean oil. I inspected the drive shaft and found normal minor wear. I'll admit i'm not an expert, but i am pretty stumped here.. any thoughts?

Ps no nail,

i just replaced the drive shaft with a stock one from a junk yard.. noise still exists. any thoughts?

check the u-joints?

the u-joints on both drive shafts appear to be fine. i didn't inspect the ujoint off the axel but from removal and install of drive shaft, it appeared ok..

the u-joints on both drive shafts appear to be fine. i didn't inspect the ujoint off the axel but from removal and install of drive shaft, it appeared ok..

check the u-joints?

I'm inclined to agree with Karl101 on this one. I've read that U-Joints specifically will cause the exact problem you are describing.

I.agree in that i have herd that too, but i have put a compleatly.diffeent.drive shaft in n have votten.the exact same noise... ill check.the u-joints on the diff tomorrow but dont remember seeing anything wrong with them.

I.agree in that i have herd that too, but i have put a compleatly.diffeent.drive shaft in n have votten.the exact same noise... ill check.the u-joints on the diff tomorrow but dont remember seeing anything wrong with them.

I would hold the driveshaft steady and try and turn the joints and see if there is any free play. Even a little bit of play can make a big difference when all the torque is going through there - that was true with my car for sure. My understanding is that most of the time the U-Joint itself looks fine, but inside where the little teeth are, if not kept lubricated it can dry out and those can break or wear out.