Hello,
I have an 08 Express 3500 cutaway (mini bus) 4.8L with 41,XXX miles on it. I'm hearing a whistle nosie when I let off the throttle above 35mph. The noise goes away when the throttle is pushed or the trans is shifted from Drive to a lower gear. It sounds like it's coming from the passenger side but hard to tell when driving.

 

check for vacuum leaks

 

I haven't seen anything yet but will definitely go over it again. There also aren't any codes set.

 

Vacuum leak in the passenger footwell area would likely be the HVAC controls.

 

With the HVAC in the off position, it still makes the whistle noise.

 

HVAC position doesn't matter. Vacuum is present anytime the engine is running.