Roadhogger-
1) Sit in the driver's seat
2) With the engine off, press the brake pedal
Does it make the noise?
3) Release the brake pedal
Does the noise stop?
If yes to both questions, that's normal. See your Ford F-53 chassis owner's manual (link
here) for details on the "Hydromax" brake booster system. It's an electric pump that supplies pressure for the brake system when the engine is not running- in emergency situations, in other words.