It's probably one of two things when your in park a) you've parked too close to the car behind or you're too near too the car in front or b) sometimes this happens to mine and it's just dirt on the rear sensors so give them a wipe with cloth and it should be fine. But to confirm it's not a fault because it does happen to me when I've parked up and I'm waiting there for someone outside a shop and I just turn it off to avoid the noise.