Thanks for your replies; your theory is correct but I haven't found
any means
of viewing or accessing the straps when a cartridge is in place. Also,
I have
been unable to find a parts list/manual for this unit.

the cartridge is probably acting as a resonator, so perhaps misleading. What
happens to the sound if you manually move the braking actuator pillar in and
out?
The other possibility is part of the slip clutch assembly is worn/misplaced
and some part is rubbing on the deck or even the underside of cartridges.