1991 Cadillac Brougham knocking type noise in engine

Yes, my mother has a cadillac broughham that sat at her brothers and was started but not driven sometimes for 3 years.I went to get the car at his house and I drove it back to my mothers which was a total of 160 miles, and had no problems with it, rode like a dream. But 2 days later went to start it and there was the sound.I changed the oil and added the valve and lifter oil stuff to in hopes that was the problem but it was not. Could you please give me some suggestions as to what could be wrong. Ty