Oye, they are a pain and they must be erradicated as early as possible! If you are a new beginner, your teacher will help you identify them. If you are more experienced you may need the help of a recording devise to identify them.
What are they?
A crossing noise is an intermediary sound between one note and another. Sometimes it's a "pop" and sometimes it sounds like a "ripple"
Why do they happen?
They are a by-product of rushing or not playing on the beat. One finger is getting ahead of the others. When you move from one note to another, one (or more) finger is covering a note hole that shouldn't be covered. When you move between 2 notes, you should only hear the note you've just played and the note you are moving to---nothing in between.
How to avoid or get rid of Crossing Noises
LIFT THEN DROP.
At any given time, the highest hole uncovered is the note that will sound. If there is any lifting to take place, LIFT the note finger FIRST, then drop. Example---D to E make sure you lift the E finger before any others drop.
If you are going down in pitch and no lifting is to take place (say High G to E) make sure G is the last finger down. NOTE FINGER LAST. In other words, don't drop the G finger, then the F finger or you will have a 'ripple' effect .