Hi Bren,

Chatter can also be caused by broken/failing motor and transmission
mounts. I'd be especially suspicious of this as you describe the chatter
happening only in reverse gear. The engine and clutch are still rotating
in the normal direction, of course, the the reaction to driveline torque
will be in the opposite direction to normal and this may place a weak or
broken mount in tension rather than compression causing a "chatter".
When I did the engine rebuild on my '74 a year ago, both of the rear
mounts had partially delaminated (it chattered in forward and reverse).
New mounts were about $18 apiece. Since I replaced the clutch at the
same time, I can't say conclusivly that it solved the problem, but the
mounts are easier and cheaper to change than the clutch.