The problem you describe sounds like a weak connection as in a cold solder joint or dirty tubes pins and sockets. The cold solder joint would be in a DIY kit so I doubt that is the problem.
As already stated move the tubes and see if the noise follows the tube.
Second get some very fine wet dry sand paper and clean the pins on the tubes. If you can, clean the pin holes in the sockets. I find a very small round hobby file works great, of course with a plastic handle. Put it in the hole gently and turn it. Not very much.
Another area is a volume control although this would show up when it is turned. Selector switch. Some say use a non residue contact cleaner. My experience is this makes them noisier. I find WD40 works great although it leaves a residue it is non gummy and lubricating.
Just some suggestion.
As already stated move the tubes and see if the noise follows the tube.
Second get some very fine wet dry sand paper and clean the pins on the tubes. If you can, clean the pin holes in the sockets. I find a very small round hobby file works great, of course with a plastic handle. Put it in the hole gently and turn it. Not very much.
Another area is a volume control although this would show up when it is turned. Selector switch. Some say use a non residue contact cleaner. My experience is this makes them noisier. I find WD40 works great although it leaves a residue it is non gummy and lubricating.
Just some suggestion.