Noisy heater on 2001 MDX

My MDX makes a continuosly LOUD clanking noise on very cold mornings when running the heater. Once the engine warms up a bit the heater works fine. It has done this on occasion every winter since we bought the car brand new 3 years ago. The dealership is no help since they cannot reproduce the problem. Anyone else experience similar problems or know what I can check for?

Maybe leaves in the fan basket?