BMW CCV Systems are known for singing a tune when the diaphragm goes bad, and creates too much crankcase vacuum.

https://www.youtube.com/watch?v=DIMpIAkWeNI

Youtube search for your engine type + whistle and I bet you'll find your noise.

What's the down side of this? I get it too, but only when I really step on it. Which isn't often

Oil consumption, smoking upon startup, fouled O2 sensors, damaged catalytic converters, testicular failure.

If the car hesitates, it could be the same thing I get sometimes, which is definitely transmission-related - almost like the trans pump is 'cavitating'.
I recently had both the transfer case and the tranny serviced/flushed and thought it was cured, but just this last weekend on a long cross-country trip, I got the howling/surging a couple of times, together with momentary hesitation, on a moderate-to-heavy throttle input.
Could this be a transmission clutch gone bad?