Try to tighten the bolts down first as they get loose easily.

 

that sounds about right a vacuum leak, the reason it goes away when you gas the truck is that your reducing the vacuum inside the intake manifold.
At idle it will be anywhere from 18 to 22 inches
Throttle open will reduce to 8 to 12 so there is less of a sucking pressure when the throttle is open thus the noise goes away.
Be careful spraying flammable liquids on the intake especially at the rear of the engine, as if you have a bad ignition wire/ leaking wire, this can easily spark the liquid to an explosion/ fire