Rattling sound when accelerating?

When I bought this car three years ago, I noticed that when the car had been run for fifteen or so minutes at regular operating temperature, there would be a rattling sound, like the timing being off, under heavy acceleration. I've had it in the shop several times, and gone through a couple of entire tune-up component changes, as well as had the timing checked each time. Everything was said to be fine. I tried a higher octane gas, fuel injector cleaner, almost anything suggested, and still the timing pinging continued, but over time, has worsened. This spring, as warmer weather has returned, the pinging has worsened to the place that, once the car reaches normal operating temperature, I cannot accelerate without bad timing pinging. I've frustrated my mechanic who's even advanced (I think) my timing to get me off his back. It doesn't seem to matter. Am I dealing with an engine that has run its course and needs to be changed?