Must have been dropped at some point, Sunn cabinets are known for being tanks.
Take everything out and off, and examine the cabinet for loose pieces and joints. Sometimes just one loose pice can really make a racket.
Get some Elmers or other construction glue, squeeze it into every corner and joint, trying to work it into the joints with a small brush. You can dip the brush in water to help thin the glue and get it into the cracks, not too much, a little water. If you can, after working the glue into the joints, put a strap or something around the cabinet, pull it tight and let the whole thing sit undisturbed for a day or two, two better than one.
Check the front baffles for tightness, sometimes these can become loose and some glue and a screw or two tightens them right up. Check all the hardware on the outside, again, anything loose will buzz and make noises you don't want.
Take your time and you should be able to restor the cabinet to it's previous glory.