He made his recording debut with the Doobies on that album in 1974, performing backing vocals over instrumental tracks that included Hossack.

[4] Knudsen organized a one-off Doobies reunion in 1987 to raise funds for the National Veterans Foundation.

Of the multiple pairings of Doobie Brothers drummers over the decades, Knudsen's partnership with Hossack lasted the longest.

On "Double Dealin' Four Flusher" (from Stampede) he is heard trading brief lead vocal lines with Pat Simmons and Tom Johnston.

(The box set Long Train Runnin': 1970–2000 has an early rehearsal version of this song, called "Shuffle," with vocals only by Simmons and Knudsen.)

He was living in Sonoma County with his wife, artist Kate Knudsen, and his daughter, Dayna Keyes, a radio disc jockey and voice over actor, at the time of his death.