new alternator belt doing noise

2 weeks ago I got my alternator belt replaced cause the old one broke, and now everytime I start my car it makes a loud whistling noise for about 15 seconds, and then it stops. now I have a pretty good feeling it's rubbing somewhere but i checked it out and didn't notice anything, what could be causing this and what are the solutions?

It probably doesn't have enough tension and its slipping on the pulley, check it and re-adjust as needed.