High pitch squeal

[scfiveo]

Hi
I have a 2011 Mini Cooper S with a sporadic high pitch squeal that happens after a cold start . When it happens if you blip the gas pedal it goes away , if you don't it will continue to make the noise , if you pop the dipstick it will stop and if you reinsert it the noise comes back . When it is making this noise if you try to pull the oil fill cap off there is so much suction you can barely get it off the valve cover . This continues as long as you don't touch the gas pedal .
What controls how much vacuum it gets at cold start and cold idle ?
I have changed the belt and tensioner , thought that was it but no luck
Any help or advice would be appreciated