I have a diesel 1.6l Ford Fiesta TDCI Zetec and recently, when idle my revs have been dipping down to 750rpm when the idle rev count is usually around 900rpm.

It dips about once a second and when it dips I hear a click sound from the engine. It doesn't tend to happen when I first turn the engine on, it'll be later on in the journey when I have to stop at traffic lights, for example.

One way I can get it to stop is by revving the engine, but this only stop it for around 30s, and it'll start dipping again and I hear the click noise.

There doesn't seem to be any problem with revs while driving, this only happens when the engine is idle and I'm not applying any gas.

Not sure if this will have any affect but I've recently had an oil & filter change.

Any suggestions or advice would be hugely appreciated.