The IWG turbos are known to have a "flutter" but it should not be making a noticable pinging noise. Its hard to explain but some noise from the flapper can be expected depending on the set-up itself. A lot of our subaru customers freak out this first time they hear it after upgrading to an aftermarket downpipe. Having said that, there are a lot of other factors that can determine how the kit performs boost wise. I always like to try and pull a vacuum reference off the manifold itself to provide most accurate reading for boost controllers/boost gauges.
Good luck tracking down your issue, the guys at GTM are always helpful but its really damn hard to diagnose something when its not there infront of you haha.
Thanks,
Alex Goodwin
AlexG@motionlabtuning.com