2000 Chevy Blazer Engine Tapping

Thank you so much for attempting to help me because we are stumped! When you start my car the enging makes a tapping noise, it sounds like the oil isn't getting to the motor. Once the oil heats up the noise clears up. We tried changing the oil pump, put Lucas Oil Treatment in it, tried a more expensive oil filter, Changed the weight of the oil and nothing seems to be working. After changing the oil it would clear up for about 3 weeks, and also when it would make the noise and even after iwhen you go on the gas the oil pressure would shoot clear up to 80-the farthest it will go, then after you go a little while it would go back down between 40 and 60. Please help!
Tara