When my Shimano right pedal started creaking, I eventually got around to checking it out, only to discover it was loose. In my naivete I didn’t realize the Shimano PD M520 “mountain bike” pedal needed regular maintenance.
The pedal was loose because the tiny 3/32″ ball bearings had been ground down, and a rubber seal had also worn down. Replace the bearings when you service the pedals, and the rubber seal, if you can find one at a bike shop.
I bought new pedals and stashed my old ones. After reading about the pedals and how to maintain them in a Bicycling magazine repair book, I knew the pedals could be serviced.
Check out my video explaining how to do the overhaul.