Whilst I agree it could well be a wheel weight........
If I were you I would immediately (if you have not already) check the wheel nuts and torque them all up!
Also try removing and refitting the wheel, its possible that you miss aligned it and its bound up in the studs, same applied that the disk needs to be sat true on the hub (could haev grit or somthign behind it)
id imagine though that your symptoms would be a bit more severe than you describe if it was those.
so lets hope its just a weight.