About frequency: if it's continuous, it probably happens once per something. Once per crank turn, once per wheel revolution, or what?
Also, it's quite difficult to accurately pinpoint where the sound comes from when you're riding. Can you have somebody ride next to you and confirm it's the rear mech? There was a case on the Forums eons ago, where a member did extensive greasing and tightening of every possible screw in the bottom bracket area only to realise the culprit was a zipper in his clothing/gear clicking against the top tube.
--J