I haven't had an ADM3 do that, but I have had it happen on Wyse 75's a lot. Use a short legnth of tubing as a stethoscope, and see what component is actually creating the noise. It could be a transformer or a coil that's resonating (check the deflection yoke), or the horizontal frequency might be running just a bit off (usually will cause picture tearing and instability). On the Wyse terminals, it was typically a small transformer on the monitor board - the laminations had started to come loose. Crimping the case back together and wedging toothpicks in it quieted them. Those terminals were sometimes _really_ loud and annoying! Or, get an iPod with earbuds, and blast heavy metal music until you can no longer hear those frequencies. -Ian