Took the car for a 20 minute drive on the interstate. Did some pulls 40-70mph and I couldn't hear the noise at all. Came back home, checked fluid level - perfect (you were right, the last time I checked it hadn't warmed up properly). Then got in the car again and pressed the pedal in P. The noise was there again at 2800rpm. Just a tiny bit more, and it disappears completely. It doesn't fade away - it just stops! I'm baffled.

I'll talk to my mechanic tomorrow, but seeing how stable and quiet everything was on the interstate, I think I'll just forget about it and wait till something eventually breaks.

 

I'd put my money on accessory tensioner pulley. The only reason I say this, I just fixed mine. Just bought it from FCP. Its annoying as hell and pretty loud outside the vehicle.