Since I bought my truck it has had a sound that can be best be described as an Owl hoot or low howel when cruising on freeway and letting off throttle to give little to no load on the engine... By varying the throttle position you can vary the howel's pitch. None when hard off throttle or hard on throttle.
Could it be a bad manifold gasket, bad boot, or cracked intercooler?? Those were my guesses. Since it sounds like air rushing past a storm door during a storm.
Thanks in advance guys!

Oh yeah,
Truck is a 99, auto trans. stock intake except K&N but been makin noise since before then.

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Sorry, I totally put this in the wrong section... That's what I get for postin from my BB!

 

Probably normal. Most likely due to exhaust gases howling as they pass through the turbine housing. Exhaust system design has a large effect on this.

 

Oh? In that case i DO have a straight through exhaust but its made noise since before i deep sixed the muffler.