Usually it is a sign that the power steering pump also is the power brake pump and there is nothing pumping when the motor is not running. There is an electric motor that will provide power for the power brakes when the motor is not running.
On my '89 E350 I would blow the fuse whenever I hit the horn with the cruise on. The official Ford fix was to install a larger fuse. The fuse was labeled horn. Check that when you are lookiing for fuses.
2003 Newmar Mountain Aire, Workhorse W22, 2008 Saturn Vue, Falcon 5250, & US Gear Unified Tow Brake
there is an electric brake that is activated when the engine is not running. if you open the hood you will see a lage elecrical looking device attached to the brakes. mine is more of a whine than a buzz. charlie in texas
1999 Southwind ford V10
Gibson hedders and exhaust
Tacoma, 4x4 toad