1997 Subaru Forester knocking noise

On start up the engine has a constant knocking noise, when warm the noise is still present and engine revs drop off. The engine still runs and is drivable but seems to lack much power its also possible that the 3rd (i think its the 3rd front right cylinder looking from the front) cylinder has a misfire. Could you please advise me to what the possible cause/causes could be.

thanks in advance

luke

ps the car in question is a uk 2.0 litre model