08 f350 with a 6.8 wheeze underload

[KHFord]

I am stumped on this truck. Customer described it as a wheeze and I have no better way to put it, can duplicate it at all speeds, just need RPM over 2500 with a bit of load, wont do it in neutral and the wheels brake loose too early to check stationary. Checked air filter and intake hose, and throttle body, no problems found. Check intake manifold runner operation and it moves freely in both directions. Disconnect and block off the PCV hose and retest, noise still present. Remove belt check all pulleys, disconnected alternator and retest, noise still present. Removed oxygen sensors and checked exhaust with borescope, both catalytic converters are intact. Definitely sounds like air, truck is all stock 137,000km. Any one got ideas to throw at me?