My right-hand SPD cleat makes a clicking sound every time I pull up or push down on the pedal, like it's loose in the SPD mechanism. It doesn't unclip itself, but it does this on all my SPD-equipped bikes, so it's not the pedal, and adjusting the pedal tightness doesn't seem to help. It's only the right-hand side, but I unclip my left foot at the traffic lights, so it shouldn't be the right cleat which has worn. Any suggestions as to what's going on?

Thanks in advance.