Have you ever replaced an H valve?
I will try tapping it like you suggest, but I wonder how labor intensive it could be to replace. I think you are right about it being a valve as it sounds very musical like a horn. Gas or liquid through a tight space would produce that sound I would think - or vibration.
It's definately not debris in the blower as I have experienced that many times - plus the heater runs quitely (except on defrost when the a/c blends in).