losing oil pressure at red light. return to normal when step on gas pedal

It's either leaking oil out somewhere or it's just plain old worn out. Make sure its full of oil and not leaking. If it has a lot of miles and has oil you might want to go to a higher weight oil like 10w40 or 50 instead of 5w20 or 30. These are just a couple of things. If it keeps going you will likely loose the engine so make sure you have it checked out if changing oil doesn't fix it.

Did you just have the oil changed? It may have the wrong oil in it. You may also have an oil pump going out. The tapping noise is probably the lifters starving for oil. If you hear a knock, its yous rod bearings.