Buzzing on a 325c58

[Tofek]

Hi,
I recently bought a Rickenbacker 325c58 and it has a buzzing problem. The B string is the only one to buzz, even unfretted. The luthier made me a new nut from bone, because on the old one the slot was too wide and the string even had a sitar effect. So with the new nut the sitar effect is gone, but the buzzing is still there. He doesn't know what to do with it. The guitar has a Bigsby bowtie bridge, which I put on to eliminate the buzzing which I thought was caused by the roller bridge but in fact it wasn't. The neck is perfectly set up according to my luthier, straight with a little underbow.
Could you please advise me on what to do, or what might be the cause?
Thanks in advance.

I can provide pics if it can help.

Here's a sample of it :

http://stormakers.free.fr/Krystof_files/buzz.mp3