I get something very similar on my PowerBook (only 2 months old). It's not related to any physical activity (like typing, etc.) -- at first I thought it was a clock ticking -- it is _very_ regular but somewhat more than a second between "ticks" -- it sounds much like the hard drive sync'ing, although that doesn't seem to be what it is. Your explaination sounds logical (I don't think I've noticed it in battery-only mode either... will check that!).

Interest to hear any solutions!