I sent you a PM earlier but I just had another thought.
When you activate the brakes, you're activating the electromagnets at the wheels. I think you're talking about your InteliPower converter (I don't remember seeing an inverter in your trailer) and that's not that far from the wheels on your trailer, right? It might just be some sort of hum or vibration from the electromagnet in one of the brake drums.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill