To be honest mate I couldn't really hear it but that may be due to the fact that everyone here is still asleep and I've got the volume right down lol.
I know my power brick is slightly noisier than the 360 itself but I'd have to get right near it to hear it, you can just hear the fan making a slight whirring noise, best way for me to describe it lol, oh and I really wouldn't leave the power brick standing
on a carpet ;)
You do get some noisy ones that's for sure, the one I got with my Gears console certainly was. Thankfully i still had the old power brick (from the old style xbox 360) those things really are silent. So with a very cheap adaptor that allows you to connect
the old style power brick to the new slim I now have the best of both worlds
A very quiet xbox 360 with a very quiet power brick