Posted March 24, 2025 at 13:00 GMT
If you're lucky, it may just be the foam behind the driver. There's a sheet of foam that is pressed by the plastic cover behind the midrange and tweeter to provide pressure form the rear to hold it in place. This foam can age and deteriorate, so the midrange isn't being held in firmly, and can rattle. Change the foam or add a bit more in, and this may well fix it.
If you're unlucky, it may be the same problem i had with both my MS150 midranges, where some idiot re-capped the crossovers in the past and put the wrong capacitor values in the midrange circuit, allowing too much low frequency to get to the driver, and eventually causing the voice coils to overheat and fall off the formers. When you re-capped, were all the new capacitors the same value as the ones you took out?