Mazda MPV, humming noise

00 MPV , v6 multiport. Air box intermittantly emits a loud humming
noise, vacuum type of noise. Wondering if this is a common enough
headache so as someone has seen it before & knows the solution. If you
pinch off the large vacuum line going from the plenum to the intake
tube, it goes away. This appears to me to be their version of an IAC as
I see none in the throttle body & there is a valve mounted where it
enters the plenum. My main question here is if it's a bad valve or if
something is loose in the airbox. This was a thanksgiving table, 20
degrees outside, no tools, preliminary investigation & didn't have the
facility to remove the airbox, just thought someone may have some
input. Thanks in advance.