Joined: Jul 2015
Posts: 7,495
Likes: 3,296
From: NW Oregon
Bikes: 1982 Trek 930R Custom, '91 Diamondback Ascent w/ XT, XTR updates, Fuji Team Pro CF road flyer, Specialized Sirrus Gravel Convert, '09 Comencal Meta 5.5 XC, '02 Marin MBX500, '84 Gitane Criterium bike
do your brake pads extend below the rim? if so (and it Looks like they do!) then that is, most likely, a cracked, Worn Out Rim.
it sure looks like a crack in that pic.....
the tire pressure can force the cracked area outward... 25k miles is a lot for a pair of rims to endure., and tandems experience much higher loads...