Idler arm!?! All of the cmax petrol and diesels have a tensioner pulley with an integral "window" built in, no arm is fitted, the only Ford engine in my mind that employs that system is the old DOHC driven by chain!
As previously stated, it can happen to any major manufacturer!
Usually there is a noise, not much help now I know, or when you're on the outside lane of a motorway!
2010 Titanium 2.0 TDCI Panther Black.