What's that buzzing sound when I play? Some questions to ask yourself...
Is the chinrest touching the tailpiece?
Are the electronics securely fastened (for electric instruments)?
Are the fine tuners engaged so they don't rattle?
Is the chinrest tight enough?
If it's not any of theses, is the tape on your fingerboard interfering with the vibrations? If so, the scoop of your fingerboard is questionable.
Do you have an open seam?
Is you bow tight enough? If the stick rubs on the string because Its not tight enough, it will create a rattle.
It could even be a loose button on your bow - probably not, but I have seen/heard it happen.
Do you have any buttons, jewelry, or zippers that are touching the instrument when in playing position?
Beyond these questions, I would need to see it in person to identify the source.