Sprint Rs (2000) Rumble / Noisy Clicking When Riding And Clutch Is Engaged

Hi,
I have a Sprint RS 955i (2000) and it is making an annoying noise (sounds quite "agricultural"). When riding along there is what I'd describe as a rotational rumble/clicking, which disappears when I pull the clutch lever to take it out of gear and let the bike coast along. I should say that the chain is quite old and probably needs replacing, but I was wondering if it could be something else that is making the bike sound like an old tractor!!!
Thanks in advance, John (based in North East Hampshire, England, GB).