Have been experiencing engine knock at low RPM. Much more prevalent during hot weather and when the engine is under load (accelerating up a ramp). Had it in to the dealer several times and they could not ID the problem. This occurred for several years. Recently I got a check engine code of P0441 (bad purge valve). The engine knock strangely disappeared at the same time. I had the purge valve replaced free (under a warranty enhancement from Toyota) and the knock has disappeared. Dealer says they have not heard of this scenario before. Just FYI !!!!!