The engine in my 98 sti was fully rebuilt 8000 miles ago and has always had piston slap when cold (cosworth pistons) apparently normal,but in the last few weeks i've noticed a slight knock/rattle at about 2000 to 3000rpm .
It only does it for about 1 minute when starting from cold and the sound comes from the middle of the engine. i think.
I dont think its big ends as its not a heavy knock and it dosent do it under load.
If it was ends would'nt it do it when warm and under load like pulling away in second.
Has anyone any ideas as the warranty on engine has just run out and by the time i get it to a garage the noise has gone.

 

One possibility is the cambelt tensioner. This can make a noise not unlike slap and can be most noticable when cold. Removal of the passenger side belt cover and running the engine from cold may help the diagnosis by observing movement of the belt and pinpoint the noise.
JohnD

 

thanks i will try that.