MPM3510 oscillates

I have a device where I use two MPM3510A.
They both get the same Vin with +24V.
One of them converts to +3V3 the other one to +12V.
Everything works fine and the ripple Voltage is nerly zero.
Then I wanted to measure electrical fast transient with a
emtest ucs500. So I connect the Supply Voltage to the
Device and then the two Converters begin to beep.
With a oscilloscope I measured the Vin (+24V) and there is
a Sinewave about 1khz and a peak to peak voltage of 26V (Vmin 11.6V Vmax 37.6V).
If I deaktivate one of the Converters, everything is fine just if both are working the
beeping noise is there.
Does anyone know how I can fix this problem so I can do my measuring?

Thanks in advance


every regulated DC-DC converter is a negative impedance to its supply. If you add impedance by inserting a test unit, you can increase the input impedance up to system instability. Most times some lossy capacity like an electrolytic capacitor added to the supply input will solve the problem.