Welcome to Beoworld !
There are two flexible wires running from the cone to the metal
chassis, ending in solder tags (that the wires to the crossover
are soldered to).
The solder tags are riveted to the woofers metal chassis and
isolated from the chassis using fiber washers.
The rivets crack over time due to stress and the inner
ring has a tendency to slide down the flexible wire causing a rattling sound when playing.
You need to push the ring back up to the chassis and glue it to the remains of the rivet.
Often, you'll also find that the inner isolation washer has followed
the rivet ring.
Make sure to keep the isolation intact (center the rivet).
Dismount the woofer from the cabinet, have a look from behind and
you'll see what I mean.
Note; The surround on some woofers protrude from the chassis and these
woofers will NOT like to be placed magnet up on a flat surface
since this will put the whole weight of the woofer on the surround.
The above applies to Beovox S60, S75, M70, M75 and others.
Martin