just cuz u have oil doesnt mean the engines ok... get a mechanical gauge, somehow.. and hook it up to where the oil sending unit is....
if you're running with low oil pressure and you hear a knock, chances are you threw a rod bearing.. which means rolling in new bearings, or worse, depending on how loww ur pressure is and how long you've ran with it that low.
could cost you new crankshaft, new rods, who knows...
DONT start your engine again until u get a mechanical oil pressure gauge on it, pressure should be around 45-60psi at 2000rpm, if any lower than 40, my suggestion is pulling the engine and checking out your bottom end
i too am sorry for the bad news but low oil pressure is no joke
if you're lucky, it could be a lifter not getting enough oil to it, which probably didnt cause any damage to anything... yet, but as stated, dont run your engine, unless you've got major $$$ to put into all new internals