the groove in the nut could be off. In addition, some strings themselves tend to squeak, a thicker gauge might help, like the Westminster E. Also, if you play really softly but don't put enough speed on the bow, it can squeak. A bridge that is too low can effect the sound. So many things, you need to have it examined by a professional, and i'm sorry, but many teachers won't be able to figure this out. You need to have it seen at a violin shop.