When my car is cold (Obviously i dont ever give it stank when it is cold) I have this slight knocking sound coming from the engine bay. I popped my hood and cant see or source the sound. My first idea was maybe its a spark plug or something wrong with the cam belt? But my plugs were new in last service and as for my cambelt, it was done 5000k's ago. When the car reaches midway on my temp guage (Thats the temp it always goes to, never been higher) the knocking stops. If i go over 3000 revs when the engine is almost warm, the knocking stops. Help?

Engine is a 4sfe - 1.8 litre curren engine.
Anything else i should be thinking about?