Unknown noise with slight load on engine

1995 Thunderbird LX with 4.6 V8, automatic, ABS, 4 wheel disk brakes, 88,000 miles, no rust, original owner, garaged, not parked out on street, regular maintenance performed on timely basis, mobil 1 oil since day 1 of ownership, no burnt trans or PS fluids, check engine light off, no codes in the scanner. Trying to figure out / find the source of a noise that only appears when driving with a slight load on the engine, goes away when light load or heavy load. Almost sounds like engine ping or catalytic converter heat shields rattling. No change in tach RPM when rattling happens, no loss of power. No change if traction control is turned off. Only other problem I am seeing is low alternator voltage according to cluster gauge with A/C on or headlights and heat on at idle, battery is 6 months old. Hoping this isn't a water pump or timing chain tensioner issue or some unknown auto transmission issue. Suggestions appreciated. - Steve