Sometimes it can be hard to hear if a particular driver is having a problem just by putting your ear up to it so I suggest the following.
If you haven’t already, rule out that it’s a loose dust cap on one of the midrange drivers. I had this problem with a new M22 and it only happened on certain frequencies. Play a scene with the offending buzz and hold your finger on the dust cap of each driver in turn and see if it eliminates of at least muffles the buzz. This would be my first guess since it seems to be an older speaker.
You could also check the mounting screws on the drivers to make sure one of them hasn’t come loose.
If that amounts to nothing it could still be something else wrong with a driver like Randy suggests. So one at a time I would remove each driver and disconnect the wires starting with the midranges, since it seem to be a lower frequency from what you’ve said. If the problem goes away when you disconnect a particular driver then reinstall that driver in another position and see if the problem follows the driver. If not put another driver in the offending position and see if that driver has a problem. If the problem is with what position the driver is in regardless of which driver is there then look for something near that driver that could be vibrating including a loose wire on the crossover.
If there is nothing at all wrong with the drivers or the position then it could be a bad crossover. I had that problem with a new M2 when only a certain frequency was played but luckily those come in pairs so after eliminating all other possibilities I swapped the crossovers and the problem followed the crossover.
