Marshall 1931 1x12 cab - buzz

[Guitarman555]

Hi, my 2 cabs (1x12) have buzz when I put them through Marshall PA 20. Both are equipped with 1xG12 H - T 1217 from 1972.
The cabs are
Marshall 1931 A - original cab in good condition, from cca 1990
Marshall 1931 copy

Both of them buzz when the volume on Marshall PA 20 is around half way and higher. Anybody has an idea how to solve it? Or can it be the speaker (speakers looks in great condition for their age, almost intact)....?
Note:
Sound si perfect, buzz is not loud, but you can hear them when the long notes are dying out (especially empty strings or power chords on the bottom). it is definetely not a big issue, but if I could reduce the vibrations just a bit, woud be cool.
When I play under half way volume, there are no vibrations. But I wanna play even louder!

[sal paradise]

Sound works through vibrations… can you be more specific?

[Guitarman555]

Sound si perfect, vibrations of cabinets are not loud, but you can hear them when the long notes are dying out (especially empty strings or power chords on the bottom). it is definetely not a big issue, but if I could reduce the vibrations just a bit, woud be cool.
When I play under half way volume, there are no vibrations.

[fisonic]

You might have a microphonic valve picking up acoustic vibrations.

[JSett]

You might have a microphonic valve picking up acoustic vibrations.

Possibly. Try with the amp off the cab to the side, maybe pop it on a soft cushion, and see if it does the same thing.