1999 Dodge Caravan Rumbling feeling since tranni rebuild

Hello Gentlemen: Since I had my tranni rebuilt on my Caravan, I have felt a slight rumble when driving at low speeds coasting downhill at 12, 24 & 30 mph. Once I pass these speeds, the rumble goes away. Other than that, it shifts smoothly and makes no noises even when the rumbling. It makes no difference whether the engine is warm or cold and this rumble only started after the tranni was rebuilt. Your expertise would be most appreciated.