when i'm riding, especially over bumpy roads, i can feel/hear some rattling around the headset area. i know for fact it's not the stem/handlebar because i've already tightened them. also, when i lift the front wheel and try to shake the headset up and down (by holding onto the handlebar), it moves up and down very slightly (very little).
is this something that is normal? on smooth roads i hear no rattling.
Yes, threadless headsets need about 1/8" (3mm) gap between the top of the stem and top of the steer tube. The gap is needed to ensure you can pre-load the bearing system without bottoming out. Anyway, sounds like it's fixed.