Had my 02 wrx uk model for about 4 weeks. The 1st week of having it not a problem. I put in some holts injector cleaner in a full tank, and a couple of days later my cel flashed about 5 times and disappeared.
So booked it in to a garage 5 days later, during this time nothing happened until i was travelling to the garage and it flashed again but this time stayed on.
when it ws checked the problem was caused by a missfire on no 1, advised to change plugs and wait to c if it happened again, but it did 2 days after i had them fitted, flashed 5 times went of, 1 day later went of again and stayed on, but just got in my car tonight and its gone of by iteself without resetting ecu, there was a slight tapping noise coming from both side of the engine bay, like a tappet noise, it seems to be louder on cold start then go faint when the light was on but hardly here it now since light went of. Is that to knock sensor cos theres a prob with engine or is it a sticky valve r nacked injector, the noise appears to be coming from under the intercooler, if light has gone of has prob gone? or still have a dianostic on it. There has been couple of mods such as ninja ii headers, 4.5 inch afterburner back box hks dump valve fitted, pipercross panel filter

 

suspect the CEL is being caused by the replacement headers- with no cat in them. you do mean headers and not downpipe? i bet it does it on a steady cruise?

knock sensor doesn't make noise- its effectively a microphone that sits on the block listening for 'knock', 'det', whatever you want to call it.

tapping noise is probably tappets, or expansion / contraction of headers.