Noticed it only happens when I'm getting deep in the throttle or going pretty fast... it also gets louder as the engine revs higher. It doesn't change with different blower speeds, but it goes away when the heat is full on. Not losing water, no coolant smell, heater works great... IDK what it is but its annoying....

 

Have you ckthe o-ring gasket on top of thethrottle body openning? Or maybe a blowen intake gasket, this was a big problem with the V-10's.

 

I'm actually pretty sure that the o-ring on top of the throttle body is gone, and I haven't checked my plenum gasket. I just changed the oil again yesterday, and noticed that it was about a quart low, and it's not leaing under the truck.... so I really hope I don't have to do the plenum gasket. Thanks for the info.... I'll look down into my throttle body in a little while, and hopefull it will b dry.