There's a mouse in my derailleur
I went for a 15 mile ride this morning. On the way home the RD began making a high pitched squeaking noise. I think it is coming from either the jockey or guide wheel. The noise is not constant, and it is most prevalent when I'm on the small chain ring and the next-to-largest rear cog. I'm running a six-speed cluster with Shimano derailer from the late 1980's.
Naturally, the noise goes away if I'm not pedaling. Anyone have a tip on isolating the noise?
Sorry about the spelling in the title.