audible "click/knock" heard from passenger side inside cab

i have an audible click/knock that i can hear inside the cab of my 01 tahoe, but barely if i can hear if i open the hood and rev the engine. i can't hear it in the engine or transmission if i use a stethoscope, but if i take the stethoscope and put it on the exhaust pipe/manifold i can hear it, even more so if i put it on the heat shield. there is the same kind of sound under the valve cover, but doesn't sound like a rocker arm. i know what those sound like from gen 1 sbc's. but i did hear clicking from the injectors when i put the stethoscope on them, but the driver's side injectors did the same thing, just no sound from the driver's exhaust like what i get from the passenger's side exhaust. the only mods that i have on my tahoe are p&p on the tb, k&n drop in filter, and electric fans. and this sound didn't start until after i pressure washed the engine a couple weeks before i did the p&p tb and e-fan conversion. is it possible the injectors are going? or possible sticky valves? i saw no carbon tracks indicating leaks, and the sound under the hood gets harder to hear with the rpms, but the clicking gets faster and slightly more noticeable in the cab. oil pressure is great