You can do a few easy tests to try and locate it. firstly does it stop when you apply the brakes? in not it probably isn't brake related. Does it do it if you declutch? if so probably something in the transmission. Does it only do when moving? if not its engine related.

OK definatly brake related then, the only thing that the drivers side has that the other side hasn't is the centre driveshaft bearing, but I can't see why braking would affect that. Does the pitch of the noise increase with vehicle speed or is it constant?

Waryscat, I've had what sounds suspiciously like the same problem with my TDCi 130. The fact that I've had it for nearly a year tells you the source of the problem is far from obvious and a solution...well, two separate garages haven't found one yet.

Mine is like an intermittent chirupping noise which usually starts up while I'm driving along rather than braking, as though something somewhere is rubbing. You can never really predict when it's going to happen, some journeys there's nothing, others it squeals away like a pigeon being throttled. I've had all four discs changed, so many different sets of pads I can't remember, but it while it sometimes stays away for a few days it keeps coming back.

Though it doesn't rise or fall with road speed, it must be something rotational but it's too high frequency to be a bearing and just doesn't sound/feel like a driveshaft or joint.

Garages just scratch their heads at me now because they can find nothing wrong with the car anywhere. I'm going to try a hearing aid specialist next...

Hi, i ran a series 3 as a cab for 300k, i had a similar problem for six months it turned out to be the handbrake cable was very slightly sticking on the nearside rear, as the dual circuit breaking works diagonaly it caused the front o/s caliper to drag only a smidge, which as it ran over distance say 60 miles sqeaked and silently cook the grease out of the wheel bearing. After having replaced two bearings in six months i finaly got the loan of a 4 poster and checked both hand brake quadrants on caliper neither was fully returning to their stops the n/s being the worst this causes the fluid pressure on the opposing front caliper not to return fully and keeps the brake pads to close to the disc, a bit like resting your foot on the pedal. anyway i changed the cable £30 problem solved. i to put new discs on it solves it for a bit until the heat throws them out of true again this proves the need for maintaining the correct running clearances when hot

Mine had the same problem around Christmas when the weather was real cold. It turned out that the NS Rear Caliper was seizing intermittently. This apparently is a common problem with the Mondeo's and is also the handbrake cable sticking. Hope this helps.