have you ever had the charger serviced in your ownership, or have any history relating to any charger rebuilds.
if you havent, now is probably a good time to split, inspect and replace all bearings and seals. luckily its easy to do, PeteG40 sells all the rebuild gear and has even put up a guide showing all required steps!
but id be doing the following
-check belt tension
-remove belt
-spin engine by hand listening for any unsettling noises
-same with the alternator
-same with the charger
-check the torque of the bolts holding the crank, alternator and charger pulleys
i'd be whipping the charger apart to have a look but then
refit or replace charger belt(s)
tension belt with a friend and a breaker bar on the charger plate 1/2" drive hole
my toothed belts have become slack and caused a slapping noise, the alternator has come loose on its mounts and knocked against the ARB mount,as well as causing the belt to loose tension!
good luck!