[klondike98]

Bob, it looks like your 610B is also an unrestored original. 

The speaker noise is definitely mechanical.  When it starts buzzing I can stop it by pressing on the cone.

It was unrestored and original before I worked on it.  I refinished the cabinet and stuffed the bakelite block caps and eletrolytic cans to keep the underside looking original.  My speaker was in excellent condition so I did not have to mess with repairs on it.  

I have had to do a recone on an 84B I worked on.  If I had to guess, based on your symptoms, there could be: a) something loose in the mounting of the speaker to the cabinet, b) a loose section of the cone at the surround, c) a tear in the cone or d) the voice coil is not aligned and is hitting the sides of the speaker frame when volume gets loud or hits the right frequency.  A close exam of the speaker while its operating out of the cabinet might help locate where the problem is.  

Others might have some ideas as well.