I have a 2005 F150 and recently have two weird sounds that seem to be coming from the brake unit on the fire wall. While driving including in reverse the sound happens infrequently, maybe every other day. A fluid surge sound is followed by a thud/thump sound. The thud/thump sound usually happens twice and the fluid surge continues until you pull over and stop. The brakes seem to continue to work properly while this is happening. If you sit for a minute or two, the sound quits and you can drive normal again with no sound.