Noisy rear derailleur (Shimano RD-5800)
Most of this is included in the comments...  I'd start with derailleur alignment.  Once that's done, you'll need to do some light tuning on the cable tension and limit screws to make up for any changes.  If that doesn't fix it, maybe a new chain would help.  Try borrowing a different wheel(with the same number of gears) to see if it still makes the noise, that will eliminate the cassette and hub body from possibly being the culprit.  I know you're saying it's coming from the derailleur, but it's sometimes difficult to tell when all these parts are so close together.  In the long run, though, if it's noisy, it may just be noisy.  If it's functioning right, it may not be worth messing with.