that is most likely your air pump doing a "self test" my 97 c230 ( c2durty) does it only at stop lights where you'd think it is gonna "die" but it never has for the last 20k, i bet it revs down to like 400rpm for a few seconds and then right back up to 750 rpms. On a nice day with windows open you can hear the air pump clicking from the front of the car. hope this helps.