Technical Punto 55sx engine issue

[simon0806]

Hi
I have a fiat punto 55sx P Reg and i recently took it for its mot and passed
since then when i start the car from cold or after it has been sitting for a while the engine revs upto to about 1.5 to 2000 hangs then gradually comes back down to 1000 als the engine does not seem to sound as quiet as before and every so often i will get a knocking noise coming from the engine
any ideas as to what causes this and how do i stop it from over reving on start up

Simon0806

 

[dave]

checked your oil?

 

[simon0806]

my oil is fine any other ideas