Not true... The ABS system comes up at startup & will activate at anytime, not just above 15mph. It'll kick on to keep the system pressurized whenever it falls beneath a certain threshold. Some rigs have more bleed-back than others, so it can happen more frequently & some are simply noisier than others.

Not that I inherently believe what the stealership says, but I too was paranoid about the sound... 3 of them have said the exact same thing re: the wifey's rig after inspecting the sound - Normal & within spec. Her's does it after every 5th pedal application; Has for 4 years (67k mi) without issue.