Just found these entries on this site
good luck with finding the cause.

Q. My violin has a dead note. Can this be sorted?

A. A 'wolf' note is what you are getting. It cannot always be totally eradicated and all instruments have a wolf note to some degree. They occur more commonly at B to B-flat on the Violin and B-flat to C on the Viola also on the Cello at E to F-sharp. They are caused by excessive tension or by a breakdown in acceptable pattern of vibration. Specialized adjustment of your instrument is my recommendation.

Q. My A string buzzes. What causes this?

A. There are many reasons why this could happen, here are just a few to look for. String wear from the string core – replace string. Top nut groove wear – replace with new nut. Fingerboard ruts and string wearing grooves, uneven surface – Re-hone fingerboard. Rattling points on the instrument body including: Tailpiece, tuning adjusters, loose ribs, ill fitting bridge foot or fingerboard becoming undone. This is a short list of possibilities and there are more. It's a system of elimination that is required and a lot of patience sometimes.