Ok, when I pass the bow in one of the strings if I focus myself I can hear a strange sound (like the sound when you pass the nails in a green blackboard). At the beginning, I thought that it was because the bow didn't have enough rosin so I put more. After a couple of minutes I could hear the sound again so I cleaned the strings with cotton and alcohol (my teacher told me to do so) but after a while, I could hear the sound again.
If you don't pay too much attention then the sound it's undetectable.
What could i do in order to prevent this?
NOTE: I'm a beginner.
NOTE: When I say in one fo the strings I mean in the E string (sorry)