rattle on tick over until clutch pulled

[unfazed]

That is most certainly not your normal Fazer clutch rumble and they definitely do not all sound anything like that. :eek
Buy a clutch cover gasket, put the bike on the side stand (saves having to drain the oil) and take off the clutch cover and have a good look at the clutch, Sounds like something broken or badly worn.
If necessary take out the plates and check everything inside there. You could have a loose of broken clutch bolt.
If the clutch was changed there is a good chance  the clutch bolts were over tighten and one was stripped.