Sorry I didn't see this thread before... I know that knocking sound well.

9x/10 its the seam where the rim is joined smacking against the brake pads. Its such a small defect and happens so fast that you can hardly see it, even if you try hard. If you spin the rim slowly I bet that when you get to the seam you will see is pretty close to the brakes on one side. If not, forget I ever wrote this