They may well be electric assist, thus you may be hearing the electric motor. Take a look at your master brake cylinder and if it has an electric motor attached, that is the sound you are hearing.
2003 Winne Adventurer, Ford F53 V-10. Toad:1992 S-10, Road Master Falcon 2 Tow bar, Blue Ox Patriot brake system and Protect-a-Tow. Right is right even if no one is doing it; Wrong is wrong even if everyone is doing it--St Augustine of Hippo. Navy (ret).