You can generate a varying sine wave using HZOSC.
Example:
(setf depth 0.01)
(setf rpm 22)
; convert to Hz
(setf speed (/ rpm 60.0))
(defun gen-vardelay (hz depth)
(setf max-variance (/ hz 2.0)) ; Max variation of speed in Hz
(setf variance (sound-srate-abs hz (noise)))
(setf variance (mult variance max-variance))
(setf variance (force-srate *sound-srate* variance))
(sum depth (mult depth (hzosc (sum hz variance)))))
(let ((vardelay (gen-vardelay speed depth)))
(multichan-expand #'tapv *track* 0 vardelay (* 2 depth)))