Strange noise when engine is warm

First a "Hi to you all!" since i am new to this forum :)

I have a BMW 98 316i compact with a strange problem.
When the car gets warm and after around 20-50 km the car starts to squeal from somewhere in the front. This sound stops when i increase pressure on the gas pedal (or increase speed). When i idle it squeals the same. If i turn of the engine and let it cool down the sound is not there when i start the car again but return after a while.

The mechanic have replaced the AC belt and Fan belt including the parts connected to the belts (i dont know the words in english but in swedish they are 'Spännare' and 'Rulle'). But the squealing noise is still there.

I would be very thankful for any help :)

/Lucas Briggs

The mechanic solved it :)

It was a leaking membrane which caused the noise.
Other symptons caused by this are uneven rpm when idling.

/Lucas Briggs