Steve,
It sounds like you are talking about a noisy 12 volt vacuum pump. The dash A/C system in your coach uses vacuum to operate the doors in the system. When the vacuum in the system drops due to a slight leak or a commanded door movement, the pump turns on to replenish the vacuum supply. The vacuum pump is located on the firewall (accessible from under the coach, between the dash and front cap). You can identify it by adjusting the A/C unit to make it come on and feeling for a black cylindrical unit that is 1 1/2 to 2 inches in diameter and 6 to 8 inches in length.
There is one strange thing if the vacuum pump is your noise. The vacuum pump is normally wired through the ignition switch, and therefore it should not run unless the ignition switch is in the run position. If the vacuum pump is the noise and it is supplying adequate vacuum, but it is a little noisy you can just rewire it through the ignition switch so that it only runs when the engine is running so the noise will not be as noticeable. Even if you replace the pump, I would still wire it through the ignition switch.
The only other thing that I can think of that could be as noisy as your complaint is the electric air compressor for the air leveling system if your coach is so equipped, however it only runs when the air leveling system is turned on.
So in reality I can not think of anything that totally matches your complaint unless your vacuum is wired incorrectly. Unplug the vacuum pump and compressor (if equipped) one at a time to verify the noisy unit. If the leveling system air compressor is the source of your noise, repair the suspension and leveling system air leaks.
Gerald
