The episode: “Secret Agent Super Dragon,” ep. 504
The riff: Sung by Tom as the titular secret agent prowls through a room and a jazzy cello-plucked “bum bum duh dum” tune strikes up.
The explanation: These are lyrics to the eponymous Tom Waits song “Diamonds on My Windshield,” from his 1974 sophomore album “The Heart of Saturday Night.” The cue for the riff was the cello–Waits’ song begins with an almost identical jazzy beat. The only way you could understand what this riff was about is if you were very well-versed with the song to recognize the musical intro.
Novelty factor: Completely new to me, unsurprisingly. Seems like quite an obscure riff. Even if you did recognize the singing as Tom Waits lyrics, you still might not put together the reason for Servo singing them. Just understanding the reason for this joke requires at the very least an intense fondness for Tom Waits.