Noodling is when a person plays, usually quietly, when they have no intention of starting a song or tune. They're often practicing or tying to work something out. It's a problem because it can interfere with those who DO want to start a song getting the tune in their head. Many people noodle a little, but some do it constantly and every gap between songs is filled with notes played at the edge of hearing.

Newbcomers with good manners are very welcome most places I've been. It's consistent bad manners because a person lacks the ability or willingness to learn them that makes people unwelcome, although sometimes it's a matter of no one being willing to explain.