Subaru Repair: Engine Noise ( tap, tap, tap, tap, tap ), timing belt tensioner, piston skirts

Question
Right i'll just point it out now that i know nothing about cars.. So please help (and its also my first car)

I have this tapping noise going on in the engine, its really bad when i first start it up, and then when im stationary in town you can slightly hear it. Ive taken it to a mechanic and he said it maybe the tappitts!!! HUH???  Ive asked my mates and they say it could be the valves... when i took it for a service at 140ks i told him the problem that ive been having. upon retrieving my car back he said he couldnt hear it that much as its not a major problem he also said that it could have been from the old oil that was in the engine and said to me that they have oil changed it so it should take a while to wear in.. im still confused!!! My car is a 1996 impreza wrx sw

Answer
The are 2 things that can cause this noise. The first noise is called piston slap, and unfortunately is normal for some subaru's. The only fix is to replace the shortblock. The other noise can come from the timing belt tensioner. The piston skirts have been shortened to allow for better performance, but the cost of this was the noise.