Before revision: In both old and new designs, even mostly directly-coupled operational amplifier circuits, feedback through the power supply rails can generate ultrasonic oscillations that vary in amplitude at a low frequency (squegging) due to the power supply voltage sagging as oscillations build up (the long time constant coming from the power supply reservoir capacitor) in such a way that the low frequency is audable even thought the high frequency fundamental is not.
After revision: In both old and new designs, even mostly directly-coupled operational amplifier circuits, feedback through the power supply rails can generate ultrasonic oscillations that vary in amplitude at a low frequency (squegging) due to the power supply voltage sagging as oscillations build up (the long time constant coming from the power supply reservoir capacitor) in such a way that the low frequency is audible even thought the high frequency fundamental is not.