My brother used to work in automotive cooling system service; they used to call the stuff "Bars Heat", for its uncanny capacity to clog heater cores.

Lower radiator hose? If tightening the clamp doesn't get 'er, drain the radiator and remove the hose and clean up the fitting. Also try a new clamp and/or hose. If it persists, and it's copper, you can have it repaired (soldered). If it's plastic, new rad time.