Engine Sounds Loud But In Time After Replacing Water Pump

We took the engine and transmission out to repair what we thought was a freezer plug that was leaking. Water was leaking from what seemed like the center where the engine and transmission meet. It was actually the water pump. We replaced it and the thermostat.when we put the engine and transmissiand back in the car. Itwas very loud. It sounded like a desiel . The timing is correct. We drove he car for approx. 10  minutes the loud noise continued but the engine had excellent power the sound is smooth and continues. We let the engine idle for approx 1  hour. It sound became lower but is sill there.No check enginlit sights on, all fuses are ok. Before any work was done the engine was very quiet. Can not figure why it runs the same just no leak and now the noise