Check streering rack fixings to bulkhead, steering arms, track tod ends and steering column UV joints. Could also be a problem with CV joints on the driveshafts. Check tht the CV gaiters are not split. If it is this the noise will tend to come from one side and be worse turning in one direction. Slow full lock turns (~10 mph) in a car park normally show up this problem creating a slow knocking noise. Try this when cold and hot to see if there is any difference.