Piston Slap? I learned this today. Tomos fins on the head are larger than most peds. When the piston's pin gets worn the piston will tap back and forth at each end of the stroke. This is on a minute scale, on the order of thousanths of and inch, but enough to make a tiny noise, or pinging, Like a rattling sound. The fins act like tuning forks conveying the ping/ring to their tips and in the process amplify the noise.
I have no idea if this is true, but I ruled out everything else and it's about the best answer I've gotten so far.
You may want to check to see if you have something loose that may be rattling, but I did too and can't find anything loose and the piston slap thing sounds believable. I heard this from the owner of a prominent cycle shop. I'll take his word. Not much you can do about it except replace the piston and pin, but honestly, I've been running my ped full bore and nothing has exploded, yet.
Like someone told me on here " run it WOT and if it runs fine and if ithe noise is still there, it's probably OK.