Hello guys. I'm running a 328i N20.
I just noticed this last night ... http://f.cl.ly/items/0U3n0a3j250j1L0v1N0J/Memo.m4a
From the ~7th second onward the engine is at a constant RPM and you can hear a whoom -- whoom -- whoom noise, noise that I don't event know where it comes from, but I presume it's the engine's fan/cooler (although I also think I'm wrong, heh).
This only happens around 2.5-4k RPM, standing still or moving, doesn't matter. It doesn't happen all the time, which is why I only have one recording, didn't manage to get better ones: at one point it stopped doing it, I took it for a sporty ride around the block, it started doing it again.
Problem is, why does it sound like that? It sounds ... out of balance. Should I take it in for a check?