ok I have a 53 plate Mondeo 2.0 litre petrol with 160k miles on. I changed the thermostat today, before I changed it, the symptoms were:

1) the temp gauge would always be on cold until u came to a stop in traffic, then the gauge would go to normal temp until u started going again.

2) the engine is rattly, clicky, like a tappet sound (sound like a diesel).

3) when coming to a stop at a junction, the rpm's would dip down to about 600rpm, go up to 1000, go down to 600 and then back up to 1000 or so (like a yoyo)

I changed out the thermostat today, which was broke, the two plastic tabs holding it in were broken which means it was always fully expanded. It was also covered in the usual crusty crap.

Now after the change, its still clicky and sounds like a diesel. But also:

1) the temp gauge from cold to the middle (normal operating temp) in about 30 seconds.

2) when coming to a stop it goes down to about 400rpm, sometimes even cuts out. If it doesn't cut out, it idles around 900rpm.

Any ideas what could be wrong? (sorry about the length of my post but I like to be concise).

Sounds to me like your cooling system is airlocked, you need to flush the system and get rid of the air.

Worst case it could be a blown head gasket.

is the tappet sound coming from any particular area of engine?

It seems to be mainly cylinder 4. Furthest away from the belt. Its just really tappy. Still don't know why rpm is dropping tho :(