Detroit Series 60 fuel economy issues

Ok, so I have an 06 Detroit Series 60 14L in my 07 Star 4900ex. Roughly 100k on an in-frame. Been struggling to get even 5mpg out of it for the past few months on good days. Used to be able to easily push 6+ out of it. There is a fluttering coming from either the intake or exhaust (can't tell for sure which one at highway speed) at 1/2 to 3/4 throttle and a secondary high pitch whistle over 3/4 throttle. Air cooler and whole charge system have been pressure tested. Turbo and vpod are both approx a year old and have been checked and passed inspection. EGR valve and Delta P sensor have just been replaced again last week due to failure. Both sensors in intake manifold have been replaced. Injectors have been replaced again after a couple were found to be failing. No codes showing in ECM and everything appears to be functioning properly. Strangest thing is when the EGR valve failed all these problems went away and I was up to 6.8mpg even with the engine derated and under heavier loads. If it wasn't for the derate and oil leaking from the EGR solenoid, I would have just left it be. I have to believe the problem is EGR related, but cannot figure out how.