Hey everyone
I have a 99 domani (el) with a d15b non vtec with an AT.
On cold starts I get a faint knocking which sounds alot like tappets but i can guarantee that's not the problem. I did a full timing kit and water pump and the car was driving just fine. Recently i decided to try some needle tipped ngk gp plugs for a change, and it worked fine. About 1 month in the knock started. I pulled the ngk gps and they were light grey/whiteish indicating lean burn, so I swapped back to the original and it improved drastically, however the knocking is still present but faint. It lasts about 5-7 secs after the first startup and doesnt reoccur unless the car sits overnight. Engine has 162k km and counting. Its a daily driver and i typically dont go gaster than 80kmh.
Some variables:
-My AC compressor clutch is shot. Will that cause a short knock on first startup?
-I've only ever went past an oil change interval once since I owned the car, by about 2k km.
I have a CEL stating that the emission control heater circuit has a fault so I'm thinking pcv, as recently I noticed it smokes when overtaking beyond 100kmh.
Given all the above I'm thinking an elimination method is my best option but could use some advice. I'm really hoping its not short block related. So, what are some likely causes for this dilemma?

 

Low oil pressure on start up?

 

Not at all, the oil pressure light cuts off as per normal on startup. Its become very sometimeish with the knocking, occuring once every 5 first starts or so. I can still faintly hear it.