j
When I change to another window or change significantly what is in the
currently displayed window, there is a noticable increase in volume
aabout a second or two afte the changer.

A hard drive of mine used to do exactly that, corrected for a while by a
sharp whack. The sound would also disappear when the case was off.
Turned out to be a faulty IDE connection.
Hope you're backed up. Argos are doing a nice little 40gb mp3 player for
£50 (Seagate hard drive) :-)