I had the same problem when I bought a second hand IS. I asked one of the guys at the service desk and he came out, twisted the whole wiper assembly once and that was it. Hasn't come back once in two and a half years even though I've changed the wipers. Not entirely sure what he did , but I assume it's to do with the angle at which the wipers are being held on rather than the wipers themselves. All wipers have a little play (usually less with better quality wipers) but if it's too much it will knock on the windscreen. If the angle at which they are held is out then the wind deflectors hit the windscreen with the natural play. This is obviously made worse at speed by pressure on the deflectors.
Hope this is useful...