Tap, tap, tap when the fan is on!

[wadjssd]

Hi there,

I've got a persistent tapping sound when my fan is on. Not the aircon, just the blower.
The tap is very uniform and steady, and stops as soon as I kill the blower. From the driver's seat, it's coming from somewhere over on the passenger side, quite low down, and sounds almost like it's under the footwell.
This morning, I left the engine idling and went around to the outside of the car, and it was clearly audible from outside too - low down around the front passenger door area.

Any ideas what could be causing this? I know there's an auxiliary heater housed somewhere, but no idea if this could be the culprit or not. The problem persists even when the engine is fully up to temperature and so the aux heater shouldn't be required.

Thanks
Andy