I agree with Sandy that the most likely candidate is the vacuum system. The vacuum pump is on the firewall, behind the generator slide, located near where the throttle is on the other side. It is aggravating to remove. It attaches via bolts through the wall, and you have to remove the steering column cover and peel down the carpet to get at the fasteners on the inside. Normally, the pump will run some to pressurize the tank when you first start the system. The sound from the pump is audible from inside the cab. Below is a photo of the vacuum pump and another from inside the cab where the vacuum pump mounting bolts go through the wall.