Squealing while reversing?!?!?

Hi guys,

Right ok, having a bit of an isssue at the moment.

When I reverse, I get a squealing noise, its like the squeal you get from brakes, its not the brakes as i'm not braking at the time it happens.

Doesnt matter wot speed i go, its happens. Had the car since new, around 18 months old now, its got 18k miles on the clock.

Any one got any ideas/theories??