Every now and then under acceleration at low speeds (around town, pulling away from lights, etc), the familiar Td6 engine tone is accompanied by what I can only describe as a 'wooshing' sound, which seems to respond to the acceleration. Best description I can give is that it's a bit like a loud fan sound - first time I heard it I thought it was the air-con fan working overtime.

I don't think there's any loss of power (such as is available in the Td6) and it only does it at low speeds and from time to time.

Any ideas?

I get a similar noise, but only after I've been in traffic for a bit. Very distinctive fan/rushing air noise.

Believe it's the viscous fan tighening up as the engine warms up.

The viscous fan coupling has a joint that gets slack and tight based on engine temperature. When the engine has been ticking over for a few moments the fan slows. Then when you accelerate the fan spins at full speed for a bit making a loud whoosh. It's very powerful and would probably suck your toupee through the grill!!

I would be more worried with no whoosh because that would mean the fan coupling was slipping.

Cheers guys - another mystery solved!