Thanks Tommy.
It actually sounds a lot like this... (specifically the sound you hear more clearly at the 0:09 and 0:19 marks) If the problem is one in the same, looks like it is the vacuum pump.
humfrz, the sound does seem to be coming from the passenger side. if it is the vacuum pump, i'm thinking that's a significantly cheaper job. i wonder if it's something i can tackle on my own. i'll need to see if a DIY exists on Youtube.