Fly-back dc/dc converter connect to external ground
The flyback module requires a minimum load of 200 mA. You need to ensure this load on its output or you will get audible and conducted noise. 

If you can afford the power loss and heat generation, tacking a 50-60 ohm resistor on its output will solve your problem. If that isn't an acceptable solution, you will need to ensure that the flyback has priority when operating. You could do this by sensing its status outputs and only switching on the aux supply when the flyback isn't able to supply the required power.