Engine and Rear end noise

[repeter]

I have an 05 M6 GTO. There is a moaning like noise at 2500 RPM it goes away at 2400 and 2600 has anyone heard this? I took it in and the dealer said it was normal... I am soooo pissed. I have two other friends that have the same year GTO and they have no noise. I also have howling in the rear end at cruising speeds. Everything is always normal when i take it in. Any advice on how to get the dealer to fix these problems. They actually tell me it is the harmonics of the engine i know that's crap PLEASE HELP!!! i have $2000 in mod ready to put in and don't want to have to take it in after the mods are in...

[Sarge]

Factory diff fluid sucks monkey ass. Drain that shit and get a couple quarts of Torco Diff fluid and the friction modifier...or Royal Purple Diff fluid that has the FM already in it.....you'll be amazed how quiet things get after you swap it out....