buzzing/whining noise all the time

So my car has been making this rattling/buzzing sort of whine for a while and it is starting to get worse, when I’m slowing down or taking off it will do this, and then sometimes just randomly while cruising. It is driving me insane, I’m thinking it might be the alternator but I just wanted to get a second opinion if anyone else has ever had this problem.

post a video

Check you a/c tensioner.