Motor makes weird noise when I spin it by hands.

[mattitech07]

Yesterday I noticed that one of the motors of my MA2 makes a weird noise when I spin it by hands, like if there's some sand or dust stuck inside of it, since I've flown at the beach recently this might be the cause.
By the way you can also notice from the video that the outside of the motors have some sort of corrosion and this is a common problem for me since I live quite close to the coastline , so the humidity and salty air gives me this problem with the motors (this also happened with my spark but i had no problems). But this is a problem that I've lived with from quite some time and the performance of the drone doesn't seem affected by this. But recently, as soon as I noticed this weird noise, I'm starting to think that the corrosion might be more serious than I tought.
What do you think happened to this motor ? Does the corrosion have anything to do with it ? Can I do something to clean the motors ?
Here's the recording of the noise:

 

[old man mavic]

try blowing out any possible debris with an air puffer or if you have a small vacuum like the ones for computers use that to suck debris out ,whatever you do please dont use any kind of solvent spay or WD40