The Online Etymology Dictionary has this entry for rabble:
c. 1300, "pack of animals" (a sense now obsolete), of uncertain origin, but possibly related to Middle English rablen "to gabble, speak in a rapid, confused manner" (mid-14c.), which is probably imitative of hurry, noise, and confusion (compare Middle Dutch rabbelen, Low German rabbeln "to chatter").
The meaning "tumultuous crowd of vulgar, noisy people" is from late 14c., probably a back-formation from the Middle English verb. It was applied contemptuously to the common or low part of any populace, regardless of tumult, by 1550s.
The Online Etymology Dictionary has this entry for rabble:
c. 1300, "pack of animals" (a sense now obsolete), of uncertain origin, but possibly related to Middle English rablen "to gabble, speak in a rapid, confused manner" (mid-14c.), which is probably imitative of hurry, noise, and confusion (compare Middle Dutch rabbelen, Low German rabbeln "to chatter").
The meaning "tumultuous crowd of vulgar, noisy people" is from late 14c., probably a back-formation from the Middle English verb. It was applied contemptuously to the common or low part of any populace, regardless of tumult, by 1550s.
Good trouble rabble rouser.
Coincidentally, I just learned about this: The Ring of Words: Tolkien and the Oxford English Dictionary https://smile.amazon.com/dp/0199568367/ref=cm_sw_r_apan_i_RXEAV834PRCBCCKRMF42