Last weekend I replaced my differential bushings, which made the car drive like a dream.

Unfortunately afterwards I've started to notice a howling from the rear end of the car at 40-45 miles an hour, more pronounced when decelerating but also present when accelerating.
It changes in tone with speed.

Pushing the clutch in and/or putting trans in neutral has no effect.

Steering left and right has no effect

I remember the sound being there before but it was nowhere near as loud and omnipresent. Due to these reasons I believe it is my differential. Is there anything G35 specific or just general car knowledge I can do about this short of replacing or rebuilding my differential? Maybe I could have loosened something while whacking it with a hammer and blowtorching the rear bearings? I doubt it.

 

Bad wheel bearings can make a similar sound. Why were you "blowtorching" the bearings? It could also be alignment related. Does your diff have the proper amount of fluid?

 

The noise goes away over 45 MPH though... Wouldn't a bad bearing just get louder? It's also less of a growl and more of a high pitched whine....

Loose pinion bearing preload is the closest I can find to the noise on youtube videos. I guess I could try dropping the diff again and torquing down the pinion nut to see if the noise goes away... My bet is the noise was always the same but now that the diff is actually connected to the chassis those vibrations are being transmitted to the subframe and I can hear them better.

 

Check your diff fluid level. Probably a bad wheel bearing though.

 

I really, really, really hope that's all it is because I was going to replace all those anyways.