Check for a crackhole in the accordion hose from MAF/air cleaner to throttle body. Change out the hoses to and the pcv valve. Change the hose from valve cover to accordion hose. Check for a tear in the air filter element. Look for small trapped gremlins.

Get a mechanic's stethoscope for about $20 and go to hunting on the idling motor. You'll be surprised what you can hear, and how closely you can track/locate noises. A poor man's substitute is a wooden dowel from engine metal to bone behind the ear, or a piece of vac hose to the ear, fishing with the other end.

One last idea. What about a belt rubbing under accel forces. I wonder, does it do it when you accel in reverse?