I have noticed that the vacuum pump for the truck makes that sound, maybe that's it. You notice that sound also, when the truck is first keyed on at start up, is that the sound you Hear?
'07 3400RL 2nd RTU
2 Golf Cart batteries 3,000 watt inverter, automatic transfer switch
Honda 2000 Generator, Maytag's with 220 Dryer
Stationary domed Sat. dish, LCD 12 Volt Amp gauge, LCD Tank Monitor
3 added 120 volt outlets, On Board Air; 12 Volt Viair 450C compressor
2006 Ford PSD 4x4 Off Road Lariat, Hadley Air Horns, Truck inverter with 110 volts: laptop, Color Backup camera, Firestone air bags,Husky air ride roller hitch, Awning lock
