Knocking

Car was running good. Needed a water pump, did not overheat kept check on water 100% of time. All the sudden after that once in awhile the oil light was coming on. Checked oil a little low. But time for oil change so go it changed and the oil light still periodically was coming on. Thought may sending unit or something. Well after about a week now it then started pinging on take off. Now has a more frequent (ping, knock). Had it looked at by two mechanics said to replace engine, two said to get a new (junk yard) head. Then I guess will need a oil pump and what ever. Any ideas to be able to test or check exactly what it is? I know obviously very little about cars. Not a mechanic and cannot afford dealership for a 2002.