Wow - haven't had any problems like that yet. There are two fans. Are they both going?
I don't know. I read on XBoxAddict some people had noticed the vent holes on their machines hadn't been popped out, so I checked that. I guess I should probably try to place it in a more ventilated area.
I THINK both fans are going, because the noise goes down considerably when I take the disc out, but that may just be a fan slowing down.