Freelander 1 reverse bleepers going mad

Hi folks my reverse bleepers used to work great but last weekend they started to be constant as soon as i engaged reverse ,so i cleaned them with the hose and they worked again for a couple of days now they seem to have a mind of their own they sometimes work as they should sometimes they just beep when im nowhere near anything and others they just beep constant any ideas muchly appreciated ,didnt over do it with the hose just like washing car normally .
cheers Trev

Usually a chafed wire earthing out, or one of the sensors corroded

Thanks freelance for the quick reply looks like its a back bumper off job then ,
Trev.