I found these two posts elsewhere. Seems to make sense. It only happens after I start the car and then only after I've started to drive a few feet. How do I know if my car has ABS brakes??
"If it sounds like a low moan or kind of like a mooing sound it is probably the ABS brake self test at around 8mph 1x/ign start. It lasts about 1-2 sec and you should not hear it again until you cycle the ignition or have an ABS stop."
"Additionally, the ABS (if equipped) will activate momentarily the first time you reach about 5 mph after starting the engine."
