What would make my truck vibrate when put in gear an lower when taken out? on 2002 GMC Savana 3500

Loud vibration sound in cab when in drive or reverse but not moving and gets lower when put in park. It also get louder when you turn on air.

Asked by for the 2002 GMC Savana 3500
possible issue with belt or timing chain. vehicle run in idle, all components at rest. turn air on, added stress to belt. possible worn out pully kit on one of components powered by belt. bring to a shop and have them inspected. get diagnostic run to see any other issues you should be aware of.