Could it just be the ball mount moving in the hitch. A couple other people here had complained about noise, but it was more of a clunking, a hitch tightener solved their problems. A squeaking noise when starting up could also be the trailer brakes
dragging or not releasing, assuming he was at a stop light
with his foot on the brake, then takes off when the light
turns green and the brakes aren't releasing right away. Best bet is jack up a wheel at a time and spin it while listening for noise, then have someone activate the brakes and release them and spin wheel again, check for dragging or noise.