Ominous Ticking from Monitor

My monitor (a DEC 21" CRT, older than dirt) has started to give sharp,
static-discharge type audible ticks, which are accompanied by short
distortions of picture and dimming. The picture distortion lasts for a
refresh cycle, the dimming can take a few cycles to disappear.

When I power off the screen, I hear a number of these ticks happening,
one after the other, tapering off.

Where do I begin to look for a solution? Google has not been much help.

Thanks!