Welcome to disc brakes, they squeal. My front rotor with 105 brakes squealed so loud it was embarrassing. And before anyone mentions it, yes I properly bedded in the pads when the bike was new. I tried every tip and trick I could find on the internet and nothing worked. Some things, like cleaning the rotor with rubbing alcohol, made it even worse.
I was at the point where I was going to sell the bike and give up on discs when I had an idea to sand the rotor. I got a little piece of rough grit that came in a tube patch repair kit and lightly sanded them in a circular motion just enough to scuff them up. Boom, silence. They've been quiet ever since. Even in the rain they hardly make a peep.