I had a problem with the reversing sensors on my GT earlier this year which resulted in a constant tone when selecting reverse.
In my case it was a design flaw, I had hit a few nasty potholes in Nottingham and one of the sensors had worked its way loose. The sensors are mounted through the bumper and into a metal bracket in a little plastic cuff. If you hit pot holes as I did, the bumper and the metal mount behind it move in different directions and the cuff snaps very easily. When I took mine to Bauer Millet to have it looked at, the service manager had another plastic cuff which had snapped in exactly the same place as mine, and in the same way.
Unfortunately it's a bumper-off job which amounts to a pretty big bill for what is essentially a relatively trivial problem.
If you're getting a constant tone rather than no beeps, my bet would be either than one of the sensors is loose in the bumper (as mine was), or one of the cables has come loose.
Do check each sensor to see if any of them are loose to the touch (it's also possible for the sensor to drop behind the top cover so it may not be instantly obvious that it's happened), grasp each one and give it a very gentle twist and pull to see if any of them come away. And at any rate, you're almost certainly going to have to take it into a garage sadly..
For what it's worth, you're going to be quoted around 45 minutes of labour to remove and replace the bumper.