The car is a 1997 323i Saloon with air con, and the fan has just started
making a low frequency warble/vibration that varies with motor speed
setting.  It seems to work ok still although my partner thought she
smelt burning when it started.

With the engine off, it also seems to vary speed up and down a bit, but
I don't know if it did that before.

The sound continues all the way through the motor speed range.  I'd say
it was about 2-10Hz.  Anyone know what the motor RPM is?

It sounds a little like there is something flapping regularly in the
airflow, like the pollen filter.  It did need changing so I'll pull it
out anyway.  Is it more likely to be a broken/missing vane on the blower?

There was also alot of leaves on the intake grill, but clearing them
made no difference, and I popped up the intake a little to check inside,
seemed pretty clean and I couldn't see any leaves.

I'll attempt to get the fan out and check it in the meantime.

Anyone any experience of a fault like this?

--
Tony