In normal running condition the car runs perfectly fine, bumpy road or not. But when driving while it is raining, even on the sweetest road, completely leveled/flat road, I hear a continues thud sound (like shaking a shoe box with a stone inside) concentrated just under the hand break area, and I can feel it as well while holiding the gear (2nd, third, fourth, or fifth). And after just a few seconds of continued thud my engine would sound and feel as though it were towing something/climbing a very steep hill. All these disappears when I come to a stop. But as soon as the car runs again it comes back. All these when it is just raining/the road is really realy wet - not flooded. Local mechanic checked all the moving parts underneath and found no cause for the mysterious continuing sound. Please help.