I would venture to guess it's the new brake shoes folks're hearing. That plus the hum and drag of the brake magnets when the brakes are applied, which very few people are used to hearing. The TV driver won't hear the magnets but someone standing next to the trailer will. We're all so used to disk brakes nowadays we forget what drum brakes sound like up close.
If you're not familiar with how electric brakes work, applying the brakes activates an electromagnet in each drum. The magnet is attracted to a polished surface in the drum. The drag of the magnet around that surface tugs a lever inside the drum which spreads the brake shoes, causing them to press against the drum's braking surface. It's a pretty simple system, even a little primitive in concept, but it works well. Both the magnet and the polished surface it contacts will wear down over time so they need periodic checking along with the brake shoes themselves. My experience is that my brakes are noisier after my trailer has been sitting unused a while, because a little rust forms on both the magnet and the drum's interior surfaces. It wears off quickly as you drive though.
I think the brake/axle maual says the new shoes have to wear down a little bit in order to conform themselves to the shape of the brake drum. Once the brakes have "worn in" you might need to adjust the brakes to the proper clearance. Again, the manual for the axle set has info on how to do that if you feel comfortable doing it yourself. Otherwise its a ten minute job at a garage with the right tools. Back in the 50s as a kid working at gas stations I used to do it often, so you know it can't be too complex :-)
It's never too late to have a happy childhood!
Lee, WU0V, and Courtenay, N0ZDT
2011 Rockwood A128
2000 Silverado 1500 pickup
60W solar system
2000W inverter generator