Trevor
The sides of the belt guard that slide over the countershaft swing head assembly run on the rubber bushes and are a tight sliding fit.
If you look closely at the parts diagram you will see that the bushes are a "top hat" shape so bear on 2 sides of the guard sliding edge.
So the bushes do stop rattling.
The problem nowadays may be getting hold of some bushes. They can of course be turned from some plastic material.
Phil