I have recently changed to 2.0L Titanium Auto to protect my worn out left knee. Recently on my 06 2.2 Titanium X Tdci I noticed what I can only describe as a `whistle sound' when changing gear, 1st to 2nd and 2nd to 3rd, it was quite obvious and louder if I changed on a fairly high rev. The sound came with the `over run' between gears and I assumed it was an idler or bearing in the gearbox preparing to fail. I am not mechanically minded but in my car which I had had from new, and was at 58,000 miles, I knew every squeak and rumble intimately. Although I have to say I have had many Mondeos and loved every one of them, so squeaks and rumbles were few and far between.
I arranged to have my local Dagenham Motors look into it after they expressed puzzlement over the phone. Before I could get it to them the rear box of my silencer fell off, jammed and I had to get the RAC to remove it. During the interim I still had the whistle when driving and eventually took the car into the garage to replace the box and listen to the `whistle'.
The box was replaced and when the car was returned they told me they could not detect a `whistle' and sure enough neither could I. It seems that when my rear box was preparing to fall off I developed a warning `whistle' when changing gears? I don't know what caused it but if you should hear an unusual `whistling' sound when changing gears get the rear exhaust box checked before it falls off, like mine did, at 40 mph.