At first I thought I just needed to lubricate my chain, so I degreased it and just re-lubed it very thoroughly. It's still squeaking. I thought it could maybe be a derailleur pulley issue, but I noticed that the squeak only occurs when the chain is pulled forward and not when it freewheels backwards. Is there any condition that would cause a chain/pulley/whatever to squeak in only a single direction?
I understand that torque load can sometimes be a factor, so I tested it by pedaling with minimal (near-zero) force and it still squeaks in only the forward direction, so it doesn't seem to be affected by heavy load vs. light load.
The only change I have made recently was that I took out and regreased my derailleur pulleys, but they were fine for about a week until today when it was lightly raining in the morning. It seems plausible that the rain somehow messed up my pulley greasing, but I don't understand why it would be a unidirectional problem.
Thank you to anyone who gives me suggestions or ideas or whatever.