Oil sending unit

That means no oil is reaching the top of headers. Either because you have sludge build up or a bearing lost pressure. What I would suggest to do is flush your engine real good and replace oil and see what happens

If the engine codes were not cleared when the oil pressure sending unit's filter was replaced either using a code reader or by disconnecting the battery, then make sure that action is taken and the engine checked to see if the failure still occurs. The oil pressure sometimes acts strangely if the engine gets into limp mode. In fact for engines with less than 100,000 miles, limp mode is what causes oil pressure issues that so many owners report _ The most likely reason for the kind of loss of oil pressure with tapping that you mention is that the oil pump is not able to get enough oil to create the needed pressure. The known problem that would do that is the oil intake pipe seal is broken. This can be checked by overfilling the engine oil slightly and lifting the rear end of the vehicle high enough that the oil pump intake is actually submerged in the oil. If the oil pressure comes up and stays up while the engine is tilted in this manner, then the oil pump intake seal needs to be replaced. _ If the oil pressure does not improve remarkably, the problem is possible failure of the pump, the oil pressure regulator spring in the pump has failed, broken lifter or some other break in the oil galleys of the engine has occurred. If the engine is old (many miles) and the flushing mentioned in other answers does not help which it won't if the engine has been reasonably well cared for, then you will probably have to tear the engine apart since it probably has a combination of multiple worn bearings, bad mains, bad cam bearings - the engine is just worn out. _ The worn condition does not have to kill the engine if you are willing to replace the pump with a high volume pump, you might be able to regain 5 - 10 psi using 5W-30 engine oil or up to 20 psi if you use 10W-30. The higher temperature oil will cause tapping as the lifters require 5W-30 unless they too are worn badly - but the wear will not occur in a way that will keep the lifters from tapping while using 10W-30 but once the engine warms up, everything will be ok and the tapping won't be very objectionable.