That is a really strange noise, and I'm sure would be quite annoying. Can't say that I have heard of this issue anywhere else though. Does it bother you enough that you are going to take it in to get fixed at the dealer?
Now that's funny, couldn't help but laugh at how it sounds.
Definitely something that requires some internal digging, take it to the dealership and let us know what they have to say and will do about it.
It may be the fan bearings that are noisy. I had a similar issue with my 2003 Odyssey a few months after I got it, although the sound was a bit different. The dealer replaced the entire unit, lots of work as the dash had to be dissembled.
My wife just discovered the exact same noise whenever she turns on and /or off the windshield defrost button.
Did you ever have Acura fix your problem? We have only had our car 2 1/2 months and this is our first issue. Hoping it's an easy fix?
Just a guess but I would say sound is being caused by switching from fresh to recirculate air. When you select defrost, the ventilation system automatically switches to recirculate and turns on the a/c in order to remove the humidity faster.
I would get that looked at by the dealer because it shoul dnot be doing that.