Front fork

This is called 'topping out' and is generally the sound of the internal bushes bashing against each other. The general cause is worn fork internals. The worst wear probably being the damper rod parts. Possibly your fork springs could be too strong but this is unlikely.

I had these symptoms on a slimline. I changed the fork oil to 20 grade and put 200CC in each leg and is perfect now. Really made a big difference.

I'm going to experiment with a heavier Noirton next week. Again 200cc's in each leg but this time 30 grade oil.

Isn't there a fix for roadholders to stop this happening? I seem to recall kits being sold

http://www.nortonownersclub.org/support/technical-support-common/roadholder-forks

Yes Dan, Mick Hemmings sold them. Basically, when you took your Norton off the centre stand the forks would dip at the slightest resistance. What that mod in the top of the stanchion did was to take up some of that slack.

But I found that 200CC of slightly thicker fork oil in each leg also did the trick and improved handling too.