I've put up several Tech-lighting mono-rail systems, and consider myself competent at trouble shooting.
Anyways.... I installed a 2 circuit mono-rail. They are on the same circuit, but different switch legs. I ran a 14-3 to the remote 2-circuit transformer, and from there ran two 10-2 landscape wires (12 volt) to the power adapter at the mono-rail. I made sure that each of the "ribbed" wires were connected to the neutral. This is a typical install for me and have never had problems before.
The humming occurs when the circuit on the 4 spots is dimmed. The other circuit has three low voltage pendants that operate perfectly (no-hum). It doesn't matter if the pendants are on or off while the spots hum.
Things I've tried: Transformer to lighting load is not an issue. The dimmers have been swapped and replaced, didn't fix it, and they are magnetic low-voltage load type. I've tightened everything that could be tightened on the mono-rail. I've swapped the two landscape wires at the transformer thinking the split (300watt-300watt) transformer might have a bad side. I've verified the voltage. I've boosted the voltage to the spots in the transformer. I've taken each of the 4 spots off and tried them one at a time. I've taken the spots and placed it on the same track as the pendants. I've changed the bulbs to a different manufacture.
At this point I'm pretty spent on figuring what is wrong and causing the bulbs to hum. Each of the 4 spots hum either alone or together.
You knew that was going to be one of the responses.
If all of the spots hum regardless of location, I suspect the fixtures themselves. Perhaps removing the bulbs and then performing a little percussive maintenance on the fixture itself, might jar something out of a resonance position.