Burden
5. Music
a. A drone, as of a bagpipe or pedal point.
b. Archaic The chorus or refrain of a composition.
c. Archaic The bass accompaniment to a song.
I have fond memories of Rag Week in Newcastle as a small boy. My mother would always take me to see the Rag Week parade by Newcastle University (at the time called King's College, Durham, before its charter as a separate institution in 1963).
Burden in that sense is a corruption of Bourdon, one of the largest pipes on an organ which produces the lowest notes. Many's the time I have had to juggle my Bourdon whilst fiddling with my Diapason.