Weird noise

[AverageJoe13]

Hey guys! I'm new here, but have read many of your helpful posts. I just replaced the STC fitting on a 2005 F350 6.0 liter diesel. At about 900 rpm there is a growl and vibration coming from the back of the engine bay. I can only hear it from inside of the cab. There is no noise or vibration at 1000 rpm or throughout most of the rpm range. At WOT there is no noise nor under load. It's only during part throttle. The engine runs great with good power. Can anyone help me out?

[AverageJoe13]

The noise was there before the job, but it's louder now. I'm thinking it might be the down pipe from the turbo hitting the cab. It's an aftermarket exhaust system. Its pretty close to the firewall at the place i see it from under the hood. I'll check this in the morning.

["Stroker 8"]