My rear brakes were making a noise when I slowed down

More than likely this is called rotor runout. This means the point at which the rotor stops is higher than the other. Since these are new then the issue is probably the hub which is what they mount too. Either that is out of round or it has rust build up on it and it is causing the rotor to have a large amount of runout. The way to fix this is to remove the rotor and clean the hub face and inside rotor when it mounts to the hub with an abrasive pad so that it turns evenly. This video shows the job being done on a similar car but the process is the same.

https://youtu.be/zC3zETXSAGk

and

https://www.2carpros.com/articles/how-to-replace-rear-brake-pads-and-rotors

Check out the diagrams (Below). Please let us know if you need anything else to get the problem fixed.