Check the position of the soundpost.
What am I looking for?
The soundpost can be seen through the treble f-hole. It should appear to stand straight up, and it should be located just behind the treble foot of the bridge. The soundpost is not glued. It should be snug but not tight, so if you take all the strings off the violin, a sudden shake can cause it to fall. The top and bottom of the post should fit the curve of the inside of the violin exactly.
The position of the soundpost relative to the bridge is so important to the sound that the French call the soundpost the 'soul' of the violin. Some instruments sound better if the soundpost is in a slightly different position. This can only be determined by painstaking experimentation. This is best done in consultation with your luthier. If you do want to adjust the soundpost position yourself, you will need a soundpost setter. You should also be prepared to reset the soundpost. Chances are you will knock the soundpost down at least once while trying to adjust the position. This is not a task for the faint of heart, or for those lacking patience. Be careful not to damage the f-holes.