-Perhaps another bleeding (I assume you pressure bleed or use an aid, not relying on gravity bleed), followed immediately by another bleeding after a short warm-up drive (caution hot) to eliminate any possibility of air in the lines. I assume you bleed all corners (good practice), farthest from the MC first. I had to re-bleed the car to get it to feel right. There were still some bubbles in the lines for me. That, and breaking it in after a few hundred miles <- broken it in yet? I read ~500 miles but ~700 was my magic number when things finally settle to my liking.
-Did you clean the rusty hubs, making sure the surface area is even/level? Irregularities on the mounting surface (rust spots) may translate to a slightly out of round rotor. The rotor floats in-between the calipers, which is bolted to the knuckle so you may have an “ever, ever so” slightly wobbly disc against the pads. Cleaning the hub was the worst part of the job, right there next to getting the damn rusted rotor off.
-I used aftermarket Centric pad retainer and also Goodridge lines and no noise for me. It doesn’t make sense how the SS line (itself) can be a culprit. The hummmm sound you describe sounds like it is rotational and only happens when stopping at +30MPH. This would lead me to investigate the rotor/caliper/pads first sa they were things that were changed, then the wheel bearings and axle. BUT I only ran this setup for about 4K miles before selling car.
-I don’t use slotted rotors. Could this be a trait of the slots (totally guessing here)?