Does it sound/feel like your tire is "rubbing" on something...if so it is likely a wheel bearing. (Just changed two in the last month)

"Clicking" (like cards in the spokes of a bike wheel) is more of a CV joint thing and you will hear that at "any" speed.

If it's actually a "knocking" sound (like on a door)...I could guess...but that wouldn't help you much.

Jason