Did it make the buzzing sound before?
If not, I would check the ground wire and make sure it is connected to a nice, bare piece of sheet metal. Sand if you must!
Another issue could be if your rear seat backs contain metal and you have no isolation. In other words the metal from your mounting screws are screwed directly into other metal causing another grounding point. With modern amps, this is usually not a problem, but it can be in some instances.
Check to ensure that all your RCAs are connected properly and not routed directly next to the 12 volt power from the battery. Sometimes when running the RCA on the same side of the car as the 12 volt positive wire, you will introduce noise.
Another test you could perform is unplug your RCAs from the amp while it is powered off then turn the amps on to see if the noise goes away with the RCAs disconnected. If it goes away, you could have a ground loop issue and I wish you lots of luck with getting it solved. The first and foremost thing I would do is change the grounding location of the head unit by grounding it directly to the vehicle versus using the factory ground wire that connects to the harness.
If it doesn't go away with the RCAs disconnected, then I would think there is a problem with the amp OR the gains are set too high.
Keep in mind, these are just some recommendations and I may have missed a few because I am tired. Regardless, noise that is not supposed to be there really annoys the heck out of me and I will spend more time tracking it down, re-grounding, and re-routing wires than the actual install itself took!
Lastly, I leave you with a little story. I once traced a noise problem to a bad set of RCAs, so now that is the first thing I swap out when I have noise problems. You don't know how stupid I felt to literally gut the interior of the car and re-route every wire only to find that one of the RCA cables had a minor short in it.
Maybe I will buy a 2016 Mustang GT, maybe not...