you probably don't pedal in a way that causes it to click. Some people find this immensely irritating, others don't believe it happens.
6 cycles, 4 pairs of shoes. It's definitely the pedals, not the shoes. I'd try another pedal system, but that's a lot of kit to replace, and SPD is good at mud.
AFAICT there are two main sources of clicking;
1) vertical free play and
2) longitudinal free play
The former arises with worn cleats (they get thinner plus they wear on the tops where the jaws bear against them) and the underside of the jaws can wear too. Often new cleats fixes it. If the underside of the shoe wears (so that the jaws wear depressions in the shoe sole) then one is entirely reliant upon the fit of the cleat in the pedal if movement is to be avoided.
The longitudinal play develops more quickly if you have 'squirmy feet', i.e. you use the float as you ride. The jaws wear and the cleat wears. If the jaws are not too worn, new cleats fix the problem. If the pedals are worn, you can grind a little bit off the rear jaw stopper and restore the gap between the jaws to the new specification (which is about 0.5mm shorter than a new cleat).
FWIW PD-M520 is the newer 'open' design of SPD. Prior to that there were two earlier designs. The first featured low float and used a different cleat (with the same PN as later cleats, confusingly) and went until about 1997. 1997 onwards pedals (such as PD-M515) have a slightly different jaw design (mainly in the rear jaw) and allow more float with any given cleat. Both early designs have a central platform (with small upstands) that may support a new cleat over a wider area than in the 'open' design. This is bad for mud clearance but better for avoiding vertical clicking.
By contrast PD-M520 and other 'open' designs support vertical loads either by the cleat bearing against two tiny support areas on the pedal, or by the jaws bearing against the shoe sole. If the shoes are worn then even a new cleat soon wears against the two support areas; both the cleat and the pedal support areas wear.
If you get vertical clicking with PD-M520 then it is worth considering
- trying different SPDs; PD-M324 have the 1997- design and so do some others
- new cleats
- swapping the cleats left for right (the wear areas are not quite symmetric)
- new shoes
- modifying the shoe sole to compensate for the wear where the jaws bear
I wonder, have you tried new shoes & new cleats with worn pedals? Does this click? Might this sort of test help to identify the cause?
with shimano shoes and PD-M515 pedals I get obvious longitudinal clicking eventually, but something (maybe my pedalling style) means that there can be a little vertical movement possible and there isn't always an obvious clicking noise.
cheers