A very slight amount that dissipates quickly could be normal, just from moisture in the air. Any more than that may indicate damp carpets or upholstery, and you'll need to get them dried out. Your truck's A/C will remove moisture, so have the button pushed even if you're running the heater or defroster. I know, the compressor cycles on & off anyway -- but there's a chance it might engage more frequently if the button is pushed... I don't know.