It's very common for the shaft sleeves of the fan to get clogged up with carbon from the brushes.
You can often get away with a clean of the centre housing.
Tapping it may give you temporary relief, or try manually ramping the fan speed up to full and see if it will go doing that.
Easy to get at fan, it's the first thing you come to under the fresh air intake grille and shut off flaps.
Good write up on
Audipages for fan replacement, take the procedure as far as required...