Uncommon high pitched whistle

I just spent $500 having my oil separator replaced by some dumbass bmw mechanic thinkin this would fix this annoying whistle that so many of you bimmer owners have had the pleasure of incurring. This whistle is coming from the engine bay on my 2005 325i. I have cleaned the intake and inspected all parts leading to intake so i know this is definitely not the issue. I read somewhere on here that it could be the crankcase breather? Any suggestions or knowledge in regards to the "crankcase breather" and is it worth another $500 just on this part (not including labor) to have replaced??? Thanks...