driveline hum/noise/dunno

[jna]

so i noticed this when i drove in my buddies dads 2000sd. when he would let off the throttle at high speeds, it would make this noise...kinda like a grinding or a high speed rotation on a bad bearing.

now that i put the bts in...when i get into OD and let off the throttle...i get the same thing but WORSE. it is pretty damn annoying and dont know what the heck it is. i never had heard someone complain about this...has anyone experieced what im "trying" to explain???

like a loud hum...grinding or some crap

[TexTon]

I had a similar noise when letting off the gas at speeds above 40mph and it turned out to be a bad carrier bearing in the front axle.