I think that theloud noise you hear when moving in reverse is to make the people behind you aware of what are you doing.
there are 2 posibilities:
-you have an alarm that offers that feature. in this case, you should check the manual for that alarm, because usually those features can be disabled using the alarm's remote and the "valet button" of the alarm
-you have light bulbs for the reverse lights that incorporate a buzzer. in this case, just change the light bulbs with normal ones.