Noisy Tappets/Lifters Loud and constant

The car hadn't been started for months.
I started it and there was a loud tappet noise which went away within 15 seconds.
This happened 2 days in a row, but the 3rd day the tappet noise did not go away.
I noticed after this that the oil was a little low ( at the "add" level) but not empty and it is around due for an oil change but not far over.
Could anyone tell me what could possibly be happening? And can the lifters be silenced again without having to pull the engine apart?
I have filled the engine with fresh oil, but lifters noise still exactly the same.