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...