wraven

I have a 2003 Kia Rio that always squeals when I reverse. It never does it
in forward. The squeal is coming from the from wheel area on both sides. If
the car is just sitting in the driveway, not running, I can rock the car
backwards a bit and get the same sound. I took it to the dealer and they
cannot find any problem and tell me I'm hearing things - NOT! When I reverse
out of a parking space at the local supermarket I get people staring at me
and my Rio and they must be wondering why I don't fix that irritating noise.
Any ideas what this problem is caused by?