Engine knocking

My car is a mazda mx-6 1990 model 170,000 miles 2.2
liters. Recently I finished working on my car replacing alternator. Before that it had sat for a few months.
Well ive started it and let it run a few times now just to keep it lubricated. Today I started it up and it was
knocking. The sound was coming from the top left side of the engine. It sounds similar to a sound I heard before but it always stopped after I revved up the engine once or twice. I took the oil cap off and listened and it sounds like its coming from inside