I did this and now my engine is making a squeeking sound when it is under vacuum.
If I take the oil cap off the squeek goes away and if I put my hand over the hole the squeak gradually comes on when the vacuum builds.
When I shut the engine off I can hear the squeek slowly fade away as the vacuum pressure equalizes with the atmosphereic pressure..

It sounds like some seal is letting in air and acting like the reed of a clarinet or something..
Is this bad or OK? Could it hurt something? Do you think it will stop? Too much vacuum?

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06 3.0 X5 auto 103k - 95 M3 - 01 RS25 - 80 931