Loud clickn noise chevy caprice 95 4.3?

i jst bought this car from n old lady i did a mayor tune bought a new map sensor n changed the idle air valve.....now my car is leakn from the radiator n it over heats(i think it needs a new water pump) but my main problem is tat wen i star the car it still idles high n low but jst a lil n it make this clicking noise from under the car by the oil pan. im not sure wat it is.....flywheel maybe????