E36 fan making low frequency noise

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
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Tony wrote:

Actually just found it in my Bentley manual, say max speed 3600rpm, so about 60Hz, could be a rotational issue with the blades.
-- Tony
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I'm gonna guess that the leaves have found a way into the fan housing.
And, you didn't say, but I assume you're talking about the fan that pumps air into the cabin.
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Jeff Strickland wrote:

Thanks for the help, it was just a leaf. I couldn't see how it could get in through the main intake with its 5mm squares, but there is a larger opening just below the windscreen on the drivers side, I guess it got in there. It was quite hard to get the leaf out, it kept disappearing, but piece by piece I got it.
-- Tony
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Broken blade?
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