Hm, what I can say for sure, it only appears when I turn AC on; with the right rpms it comes straight with the button pressing and then like I described - every 25/30 secs.
I had the feeling, that it almost sounds like something pulling hard, but thats just one of those "maybe possibilities thoughts if the mainthought is wrong" ^^ And the main thinking is that air pop sound.
I dont know exactly when it started, but we changed the whole timing belt stuff 3 weeks ago and everything was fine before - but would that have something to do with this issue?