Marker beacon flashes

I have an old marker beacon that works normally (lights and tone),
except that the inner and outer lights also twinkle on and off
whenever there is a transmission--either on the intercom or external,
originated by me or anyone else. In other words, any communication
activity causes the MBR lights to flash on and off.

Before I take it to the radio shop, any ideas what might be causing
this?

Thanks.