My 04 wrx usually has a slight knock in cold temperatures when first started, from everything ive read this is somewhat common. The knock goes away after the engine gets to temperature. The strange part is, if i start the car and let it run for 5 minutes or so (while knocking), then shut it off and start it again, the knock is gone. I always run 93 octane fuel. Also I generally drive with the economy tune on my accesport. Has anyone experienced this?? Any info would help.