A while ago the Peugeot commuter developed a creaking/clicking coming from the bars/stem area. I solved it then by greasing the stem clamp bolt, and it was fine for a while. Slowly but surely it started creaking again, until it was getting too loud for me to ignore!
So today I loosened up the stem's bar clamp and put a large excess of copaslip on the metal to metal contact. That removed one of the creaking tones, but there was still a slight clicking, so I took apart the quill stem's expander bung mechanism and copaslip'd all metal on metal contacts as well as all threads on the stem.
Success! It's quiet again! And there's enough copaslip in there to prevent corrosion for decades.