Interesting Jedan
I have a similar problem in my 1994 e36 318, i found one hose to be stuffed so brought an new one from bmw in hastings bloody expensive vac hose it was $97. then found another crack in hose under the idle cover so repaired that using some super glue and heat shrink.... for the time being. but the car idles like crap and has a squeal thats dam right embarising. i thought it might have been my at fluid so checked and topped up but noise hasnt gone away. My belts could probably do with a change. but this also sheds new light on an on going problem so will try that in the morning and see if it works.
Thanks
Boon