Your posts suggest you hear audible noise from your flyback converter. There is a type of faulty behavior that creates On-Off buzzing, as the circuit operates okay for a few cycles, then is idle for a few cycles. It is known as 'hiccup' mode.
Or, since you state a time of 1.6 seconds, is it the IC going into shutdown, then starting up again? Often such a safeguard is built into IC's to protect against overvoltage, overcurrent, etc. However your post says it's disabled. On the other hand do you attach a load at all times? Many smps require some amount of load for proper operation. With no load the output can be unpredictable.