The Jensen 20 is a firm favourite here, and your´s certainly turned out to be real gem in the end.
Well done !
As far as the knocking goes...
If you can´t find any obvious slop in the various connections causing it, try adjusting a bit on the screws holding the crosshead guide to the cylinder.
On more than one occation, i´ve found that minor adjustments there, making the crosshead run completely true, has eliminated a knocking sound on these.