I've got this annoying supercharger type whine from the front, most noticable under engine braking at low speeds, probably because in this situation the exhaust is quiet. I suspected it was there all the time, not just under engine braking.
This noise was apparent on my test drive 18months ago, I initially thought this was a collapsed oil filter as the car it was overdue a filter change and the filter did look a little squashed.
After changing the filter very soon after purchase the noise had reduced but thinking back, this was probably placebo.
After my 400mile drive a few weeks back I thought it was time to start digging deeper as the noise was still bugging me.
I took it to my mechanic early this morning for some diagnosis. He was surprised how loud it was and commented it was louder than what he was expecting to hear from my explanation.! Oh. :rofl:
He came out for a road test, we performed loads of situational tests, but he wasn't convinced the cause until he got the stethoscope out. At 3k RPM the alternator is making a loud high pitched whine. Nice! Diagnosed!
So, I'll get an alternator on order and he will swap it over for me in a few weeks.
========================================
[Edit/update:]
New alternator installed, no difference.
I'm honestly starting think this noise is absolutely normal, 2 people have now told me their N52 sounds the same, I've found numerous videos online from people trying to identify other noises and I can hear that faint whine in the background then they rev the N52 up to 3k rpm and then decelerate.
The stethescope does indeed amplify a whine sound (as designed)
That combined with with me wrongly comparing the noise to my previous 2 N52 130i's (with probably MUCH more sound deadening than the Z4) sent me down a wrong path thinking there was a problem, when there is not.
========================================