Worn bearings tend to make noise at any speed but it is possible that the noise only rises to an audible level when at speed. But because the wheel rotation is fairly rapid at 15 mph, I would expect the noise to have changed to a screech (constant noise) and not be heard as a repetitive sound. Also, relating to wheel bearings, I would be surprised if rider weight made a significant difference in bearing operation (which is preloaded within the hubs). I would think you could eliminate wheel bearing wear by examing them off the bike. (Checking for smoothness of operation and any lateral or vertical play.)
Sometimes a wheel spoke can loosen and create noise. Even if the wheels appear to be true, check the spoke tension by squeezing pairs of spokes and working around the wheels. Also check that the wheels are seated squarely in the frame and that the rear wheel isn't touching the chain stays. Look for sidewall damage, puckering of the tire, or anything unusual.
Just some thoughts. Good luck.