Popping noise

I recently started to hear a rather loud popping noise coming from under my feet as I drive my 2001 s40. It happens at lower speeds when I hit bumps. It doesn't sound like a suspension problem but that something is loose perhaps in my transmission or drive train. I took it to Sears and they could not figure out what the problem was. The interesting thing is that when the car is off or in park, you can push the car up and down and it does not make the noise even while turning the wheel. It has to be on, in gear and moving for it to happen. The mechanic at Sears looked over the CV joints, shocks, struts, etc. and did not see any problems. I am taking it to a Volvo specialist on Monday, but I want to be knowlegeable going into the mechanic. By the way, I just had new tires, brakes(pads and rotors) and alignment performed. This car is turning into a money pit and I am wondering if I should put any more money into it. Any ideas? Thanks.