Is the knock coming from the front or back of the car? Also what sort of knocking, is it one tap or thud , is it rhythmic and related to road and/or engine speed? Have you checked the wheel bearings? Likely to be the right hand wheel bearing if it knocks on left hand corners and vice versa but say it knocks on left hand turns.
When you say it knocks when you slow in a straight line, is that by using the brakes or just letting it slow naturally?
Have you checked the wheel nuts for tightness? Also, jack the front up so a wheel is in the air, grab the wheel at the 12 and 6 o'clock positions and try rocking it, then repeat at the 3 and 9 o'clock positions - can you recreate the knock?
Could be so many things and sadly, your description is a bit vague - without hearing it or a better description, it's going to be hard to pin this down!
