I have a 99 Dodge Quad cabW/ 6 cyl. Cummings. Had the vacuum switch changed out to engage the 4 wheel drive and noticed a humming, like you would get when you bottom out your power steering. Looked for my PS booster and cannot find one. Is the PS somehow boosted by the transmission? Also get this hum when pumping the brakes. Any help will be much appreciated.

 

The PS pump is located down low on the drivers side of the motor, it's also connected to the vacuum pump.
See this pic, right in front of the oil pressure sensor.
http://www.dodgeram.org/tech/dsl/Fac...ocations-l.gif
One more thing, there's no "G" in Cummins.