[LuxoM3]

Oil seepage around the charge-pipe isn't that end of world and there isn't anything mechanical in the charge pipe to make noise.

Blow-off valve, stock or not, it's a valve with a simple motor, so nothing dramatic enough to make noise loud enough to be heard.

OK so when trouble shooting, try to stick with the original problem without going down too many rabbit holes.

"I have a strange turbine sound coming from the engine bay of my F32 2015 435i (n55) when decelerating. It sounds like a helicopter just after it turns its engines off. The noise starts at around 30mph 2,000rpm and is most audible when I’m slowing to a stop."

There's only so many things that can make noise on deceleration:
1. brakes - because you are braking
2. transmission / driveline
3. alternator - when these are dying, you can hear the bearing and/or they don't charge the system...

How many miles on your car? Automatic or stick?
And are you specifically doing anything like braking or downshifting when this noise occurs?