Strange noise from 530
I have a 2003 530i, today while parked in the driveway I noticed a very low but audible noise coming from the front when I was in the car. It sounded almost like an airplane powering up to take off, it would increase in pitch for a few seconds stay flat then lower in pitch and go away, and would repeat itself a few times. I'm assuming it was some sort of vacuum noise. Very low volume but definitely audible in a quite setting with the radio off, I noticed it again while driving the vehicle later.
Is this a normal noise or something I should be concerned about?