I've got a 2002 Ford Mondeo Mistral 1.8 bought a few weeks ago it's been no trouble and still isn't there's just concerning noises coming from the engine. After driving it (not short drives a good half hour or so) there's a hissing noise from behind the steering wheel which I can't hear under the bonnet but it's only when the engine is on. I've been looking on forums and I've found a bit about leaking pipes but I thought if that was the case I'd still hear it when it was switched off.

I don't want to go to a garage yet as last time with my old Mondeo that was overheating they flushed the radiator which was quite expensive and it just turned out to be a sticking valve which cost a couple of quid! Any help appreciated.

you have the heater matrix behind the dash.Are you loosing water ??

I would say it sounds like it might be coming from the heater matrix too. check see if the floor is damp, but if its only a hissing the floor might not actually get damp certainly. but usually you can smell it if its leaking from inside the car.

Either that or you might have a peed off snake under the dash :D