Well, there are several things that can be done. If you have a blue glow inside
the plate, it's gassy and will cause noise. (Outside the plate is okay.) If you
tap it, holding the top with a finger of one hand and flicking it, using thumb
and index finger of the other hand, look for sparking around the cathode (and
listen for a static like noise). The tube could be intermittently leaky, even
shorting. Listen for a "bonging"sound. Although this won't affect performance
too much, it -- a microphonic -- could interfere with decent reception of
strong signals.

If the filament goes on and off, it, and a static noise without a shorting
indication, most likely would indicate a bad socket contact. Short of replacing
the socket, get a paper clip, straighten part of it, and cut barbs into the
straightened part, scattering the barbs around the wire, preferably pointing
out (up) so that they scrape any crud when the wire is removed. Then work the
clip's wire in and out of each socket pin. The clip's wire should be the same
diameter as the tube's pins. Unless there's a lot of crud in the pins, spray
cleaning doesn't help much.

Sometimes a socket's pins will spread and lose tension. Take a scribe and
squeeze the two halves of each pin closer together. Clean them first, though.

73,
Bill, K5BY