Ok, I have an update! I started it up again and let it run for 20 min and waited for the sound to start again. I twisted the oil fill neck off and the sound did stop, and i then put it back on and the sound resumed. I did that about 5 times to make sure.

I went and bought a new PCV valve and installed it. The sound still comes back though. So do you think it has to be a broken vacuum line then?