[Amps] Yaesu FL-2100B problems

[José Arturo Molina]

I was recently given a very well taken care of Yaesu 2100B. The amp is 
working as it should; however, sometimes, I can hear a sound like 
"vibrating glass" and my modulation in the scope is out of focus then. 
Eventually, the problem goes away. Is this a known fault of this amp, 
could it be a tube loose at the base? Does any company offer overhaul 
kits for this amp?
73, happy 2009!
Jose, YS1MS