You have to be careful when pulling and putting tubes in. The wiggle to put them in or pull them out is problematic because it can diminish the tension on the tube pins and eventually cause a short or failure. Try pulling them straight out or pushing them straight in as best you can if you do indeed take them out.
Power Tubes - Tube age and amp volume has very little to do with longevity. New tubes fail all the time and some tubes last for a very long time. If the tubes are not broke, I would leave them alone as i doubt they are bad.
Pre-Amp Tubes - They can fail quickly or last forever and anywhere in between. When pulling them out, refer to my comment above on the wiggle.
Have you checked your amplifier power source? Is it high or low or normal voltage? A shift can alter sound radically and cause noises. Try plugging your amp into different power outlets and see if this has any effect.
Do you have any scratchy pots on the amp?
Is anything hooked up to the amp and or effects loop? If so, disconnect all of that and go straight amp and see if the noise persists. If you have multiple cables, try those as well and see if you can pinpoint any changes.
Look at your power amp tubes and see if any tube(s) are glowing differently from the others.
With a load on the amp, sweep the volume and FX loop controls (if you have one) and see if there is an increase in the noise volume.
Does this noise happen immediately upon powering up or does it happen or does it take a bit to show itself?
Does the amp make the noise while powered on and the stand by is off?
Does the noise come immediately once stand by is engaged?
The De-Oxit on the contact points is never a bad idea. Spray a little on your cable end as well and insert those into all your cable jacks. If you are comfortable, you can do so on the pots as well.
The huge majority of tube amp issues are tube related. Hopefully these suggestions help you find and solve the issue.
Good luck!
