Background:

Very annoyingly, just as your favourite loud piece of music is reaching a Crescendo or everyone is playing ffff , there's a "rattle".  Bloody distracting. What could it be?

Scenario:  The one, and only, cause of a true "rattle" is something mechanically loose.  A screw, a strip of wood or plastic. One of the grilles.  Usually, it's a grille vibrating against the frame. Check the rear grille in particular with all those little screws holding it on.  If you work up the enthusiasm, take the rear grille off for a bit. Listen to see if the rattle is gone. If not, you've lost some time, that's all.  Replace the grille but run some thin gauge foam tape around the frame where the grille attaches and this should remove the rattle for good.  If you find a loose screw, fill the hole with wood filler, drill a very fine pilot hole (1mm) and refit the screw.