Ganuun ba? I was about to suggest that maybe your sensors were loose (mine were on previous car, so it would beep on inclines)...

Masyadong reflective siguro yung pebbles. Because the sensor sends out an IR signal which merely ricochets off the road at an angle, while pebbles would reflect it straight back at the sensor. Strange na napaka-wide yung IR signal niya... try it kaya with aluminum cans and see if that triggers it, too? Does it beep for parking kerbs?