Fluttering noise under load.
 

The truck is a 98.5 24v manual. When under load and boost is coming up I can hear what I can only describe as a flapping noise from under the hood.(almost like a plastic bag got stuck under the truck or something). Too slow to be the turbo shaft but was thinking that the wastegate actuator might be bad? I am running the EDGE comp box and it makes the sound no matter if the box is off or not. If i let off and get back on the throttle I can make it go away. Cleaned the airfilter, no shaft play in the turbo itself, I'm lost. Ideas??