The noise comes from a "stuck" blender motor or similar name. It will vary with the temp control as the motor moves the flap to allow cool/warm air.

I had the same problem on my wife's 99ML. Part costs around $40 but labor is around 5 hours (not 16)

I had it fixed at the dealer and the total bill was just less than $500. I always worry about dash removal jobs but it went back together fine and no rattles as a result.

My opinion is that its NOT an easy DIY fix. I managed to see the problem by removing the glove box insert and shining a flashlight inside but I couldn't see an easy way to get to the problem. I think that the Techs remove the top half of the dash but I am not sure.

Sweet relief once fixed though!!