It's probably your rear brake rubbing against the interface between your mudguard, rack, panniers and rear reflector.
@deleted started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
It's probably your rear brake rubbing against the interface between your mudguard, rack, panniers and rear reflector.