there is a suction through oil cap. when it is on the engine makes a high pitch. on 2009 Volvo C70

when the cap is off the sound stops and there is a suction at the opening from hole where oil goes.

2 answers
The suction is normal.
It sounds like the seal in the oil cap is bad. Usually you can replace the O-ring.
If you can't just order a new oil cap, shouldn't be more than 10 bucks.
Engine breather is stopped up! Pcv system cant pull in fresh air through it.