SF Twin Reverb odd noise

[Teleman55]

I have been working on a friends SF Twin,all seemed well until it went home.The owner said it still has tube noise.He brought it back to have new Power Tubes installed and biased.While I had the chassis loose I noticed that when lifted on corner and flexed the chassis it started rumbling-this only occured while it was warming up,once warmed up it no longer happened.Any ideas? I am thinking bad solder joint or cracked resistor maybe.

[Enzo]

My first thought is failed solder joint. Isolate the problem. WHich controls have ANY affect on this noise? Controls that affect it are after its source, and controls that do not are before its source, or not in its path.