My advice is to take your violin to a good luthier. Without knowing the history of your instrument and at what point this squeak began to appear it is hard to diagnose properly. If this squeak has been an ever present problem since day one or if you have had some set up work performed recently I would guess that the sound post has been repositioned or was cut too long from the start. A sound post that is too tight will often produce squeaks (commonly referred to as wolf notes.) However as mentioned before too much rosin can be as much of a factor as too little. A host of various other things can cause the same problems. Another that comes to mind is strings not running parallel to the bass bar. Hope this helps just a little.