Odd issue with an aksium front wheel

I brought a new aksium front wheel from one of the on-line shops, put the tyre on it and fitted it to the bike. Went for a short test ride and heard a rattling noise which at the time I though was one of the cables rattling in the frame. On closer inspection it seems to be the wheel. Holding the bike with the wheel off the ground and tapping the rim lightly with my finger the wheel sounds solid, no problem, but at a point opposite the valve it vibrates making the noise described above. Also spinning the wheel an eighth of a turn at a time and dropping it to the ground from about half an inch at the valve position again it vibrates. Is it a duff wheel or what? Does it need to be ridden for a while?