That's an interesting theory ,Kevin,although I did thoroughly check scuttle area/carpet and the likes for any sign of water ingress and nothing found.When I first got the car I actually removed the rubber drain bung in the nearside wing area so in theory it should never block ,but do seem to remember a while back we had rain like the tropics so maybe that was the start of things.
Actually thinking aloud now I do seem to remember yesterday the fan did make a noise like a leaf or similar was stuck inside 1 of the vents but it did clear and with radio on I must of just forgotten about it.Maybe your suggestion of the bearings starting to dry out is a sound one.
Assume to attend to this properly the fan motor assembly needs removing or can I just aim some WD40 or similar in the general area and hope it does the job.
.Could it be accessed from the pollen filter housing