Tubes can make noises at any time, sometimes it means they're failing and sometimes it doesn't. Power down the amp and swap the tubes from channel to channel. Power the amp back up and let the amp reach operating temperature. If the noise travels with the tubes, you very likely have a bad tube. You can narrow the problem down further by swapping one tube at a time. Also turn off the lights in your room and view the tubes in the dark. If one tube looks dramatically different than the others or is spitting sparks, then you probably have a bad tube. You could also have a bad bias resistor or some other part in front of the tube could be failing. If you feel this is a tube problem, you should get it resolved quickly since a tube failure can take out other components. If it is not a tube that's failing, then you should get the problem resolved quickly since parts failure can take out a tube. If you feel swapping the tubes has had any effect on the noise, you probably need to do a thorough house cleaning on your amplifier. Sockets get noisey as the pins oxidize.

Not that I know of at this time. You might try here; http://www.partsconnexion.com/ Or try any of the tube sellers by way of a search engine. A pet shop might have a tube cleaning brush that will fit your larger tube's sockets but you'll still need a cleaning agent. Try some Pro Gold in a spray can. Radio Shack still sells this in small cans and any pro audio shop should carry that or a similar product and the pro shop might have a socket brush. That is often enough to get the oxidation off the socket. Use it sparingly and place some paper towels inside and around the amplifier to minimize splatter. If you rub too hard with Pro Gold on any silk screened printing on the amp, it might lift off, so be careful. A towel dampening with warm water should be all you need to clean smudges off the surface of the amplifier.

Remove and re-insert the tube several times while the socket is wet with the cleaner and this should help quite a bit. Be careful not to break the glass envelope of the tube or bend the pins. Handle the tubes with a paper towel to avoid getting the oils from your fingers on the tube envelope. Take a pencil eraser to the pins of the tubes to get the oxidation off. You can also take very fine (600) grit paper to the pins if you're very careful not to scratch the surface. The flint from a book of matches works also. You're not looking for "shiny" so only use the flint/paper if you must and work with a very light touch. Clean all the contacts with Pro Gold while you have the amp powered down. Make sure you get the cleaner and not just the conditioner. Let the contacts dry thoroughly before powering up the amplifier. Tube amps should get some sort of housekeeping at least once a year as should the rest of your system if you're serious.

Great info.

Pro Gold is now DeOxit Gold. I suggest the GX5 variant as it is made for high temperature applications.

A great price (Good place to buy from, too.)

It's amazing how much corrosion there can be on tube pins.

My experience is that if it is the tubes, it is more likely an input tube (the smaller tubes) than the bigger output tubes. But this is just my personal experience.