elmac

My wife and I own a 2000 RX300 2wd with 80,000mi. When we purchased the car, we were told it took regular gas...after about a month I noticed pinging and switched to premium. Temporary fix...now when driving up a fairly steep hill to our house the valves clatter. I've taken to local mechanic...they test drive and naturally don't hear any noise. Now it seem to clatter even when on flat surface during gradual acceleration. It's driving me nuts! Please, any help would be greatly appreciated! ElMac