eddie98lsx

ok guys im having problem here and so many different things can be...first off i installed a ms4 cam 239/242 .649/.609 chromoly hardened pushrods 7.425 and prc double valve spring kit. 60lb fuel injectors all new... i had all this new ready to put into a stock ls1 i had the heads ported and new valve job done to it before all this was installed, left the old lifters in and everything else oil pump too..so now on to whats happening..i start the car up and everything seems fine run it for a good 10 minutes then after it warms up for a bit it starts a light ticking noise and graduates even more the longer its running and every once in a while it will go away and then come back. now i been noticing my oil presure is pretty low compared to what its suppose to be. at iddle cold and when warm it will be at 18psi if i get into it a bit itll go up to 38psi but never over 40psi which is halfway mark. iv put oil detergent cleaner in it thinkin lifter could have gunk on it n fuel injector cleaner as well. ran 50 miles and nothing changed...i dont think lifter are bad because if car is cold it doesnt tick...im thinking can be oil pump, oil sending unit, lifters, or maybe even adding adjustable rocker arms. iv never delt with this before, im trying to get other peoples input on this...im stumped and dont know which best way to go about this...plz HELLPPP