In some sense, valves have not been perfected, and they frequently can rattle or buzz. The tape side goes down, toward the plate, which helps reduce sticking, popping, buzzing, etc. The valves should be trimmed to just barely cover the slot they're on top of. Take care when reassembling the harmonica that the comb does not interfere with the free operation of the valves. If the comb keeps the valve from lifting during play, the reed won't sound, or won't sound right.
Valved bends are a little different than normal diatonic bends.
During a normal bend, both reeds in the chamber can participate to produce the characteristic gutsy sound. Valved bends are more delicate and require more control to execute cleanly and clearly on pitch. Only one reed participates in the generation of the sound, since the other reed is blocked by the valve. It is especially important not to attack the bend hard when you initiate it, otherwise it will choke off and not sound. A pinching of the lips will not produce a good valved bend.