2001 MPV Noise from Air Filter Box

Hello,

Sometimes there's a strange noise I notice while driving the van. The other day I popped the hood and manually pulled back on the throttle. I found the noise was coming from the air filter box/case. It only happens when I rev it pretty high (manually) and sometimes while driving too. The sound is like a vibrated "wha, wha, wha, wha" sound (sorry no other way to describe it).

I replaced the air filter this evening thinking this would fix it and it didn't.

Anybody have a clue what this is?

Thanks,

Shawn