Where the front derailleur cable comes through the frame there is a hole. Pretty stupid design, as it should clearly be a mtb style rubber seal, but there it is, and it was filthy and packed with sand, dirt and sticky dried oil and Gatorade.
Here's a nice video showing how to clean that hole and the thru-frame cable guide.
You should also clean the guides for both the front and rear derailleurs that run under the bottom bracket, as being at the bottom of the frame, they get fouled by Gatorade that spills on the frame and runs downhill.
|Chain on biggest rings, shifters positioned to relax cables for max slack, makes for easy cleaning of BB cable guides. Note pedal tether to prevent shifting.|