1996 Toyota Corolla engine rattling noise

Hi, I have 1996 1.6L corolla AE102, it's been great car to me for almost 10 years. I experience rattling noise when exhilarating, but it's only when engine get's hot, that's approx after 10min, it sounds like pinging rattling ringing noise, which is the the worse at gear downshift between 70km/h and 80km/h ( it's a 3 gear automatic with approx 280 000km) I have changed the timing belt around 2 months ago that's when that noise escalated to a loud worrying level. Old belt was stretched approx 5 degrees ahead ( crankshaft was ahead of camshafts 5 deg. ) When the car is cold, first few minutes or km, there is no noise, car is smooth, quiet and strong, but transmission doesn't change gears for those few minutes, it's still 3000rpm at 80km/h. If this noise is from valves that are rattling around, I guess I should measure the shim distance from the camshaft when the engine is warm (instead what is recommended in the Haynes repair manual - at cold engine) if it's something in the transmission - it's out of my abilities. Thanks, martin.