Is it a 'droning' noise?
If it is then it's the bottom pulley. If you do a search on google for it you will find it's a VERY common problem and can only be fixed by changing the bottom pulley, tensioner and the auxilary belt. There are other mondeo forums which have more infomation but im not sure if im allowed to put a link on here to them.
It wont affect the engine if you dont change it, but it may drive you bonkers listening to it rumble away at idle. If you dont want to pay the £300-400 to change it, turn the stereo up!!!