Engine tapping

i have this issue going in another tread, but i am starting a new one with a more appropriate title,

so i rebuilt my engine, changing every bearing, seal and oil system as i had low oil pressure problems, (which were caused by the oil squirt pressure relief valves), since rebuilt i have had to replace front crank seal and cam seal again!

but i am also having issues since rebuilt with a ticking noise under load or over 1500 rpm, someone suggested that it could be the sediment filter so i have cleaned that there was some crap in it, and pressured the pipe, some crap came out of that too, refilled the system, put a clear pipe between my filter and the fuel pump, i noticed the little pipes between the injectors were damp so put some clear tubing on them too,

started it up, i have no bubbles in the pipe between the filter and the pump, however the little pipes between the injectors soon started filling up with air, not sure if this is normal or not.

still have a tapping/ticking/knocking, i got a long screwdriver out and started proding it around with it to my ear, tappets sound fine, pump, turbo, everything on fan belt all sound fine, i the checked each injector one by one, there is a distinct knocking on the second injector,

so i then swapped the 2nd injector for the 3rd one to rule out the injector, still got a knocking on the second injector, the second injector goes back to the highest point on the fuel pump,

as far as my knowledge goes im stumped, what are the possibilities?, could it be just a new diesel knock that i haven't noticed before, could the fuel pump be screwed, or head gasket.

any help here is much appreciated

anyone?

i have just cracked all the injectors, number 4 has a good powerful spray, number 2 has a dribble, and 1 and 3 are sort of in between, the overspill of diesel onto the plugs identified some leaks on glow plugs, i can see air bubbles there, im presuming i have a fuel flow problem, could the lift pump still be at fault even if there is no air bubbles going to the injector pump?

could i still have an air lock in injector pump as it was drained to carry out rebuild, or is it the pump is screwed?

what are the tell tail signs please help!

whilst driving i have a squeeking when engine is cold, this develops into a tapping (or chugging as my wife calls it), i am feeling surges, fuel economy seems ok, lack of power, and it wont turn over when its warm (this could be starter motor as it was on its way out before)