This video found on YouTube is a perfect example of a filter cap problem. On solid state units like this one, through vibration and assembly techniques, components can work themselves lose from the circuit board. The owner greg1485 from this video literally has his finger on the problem.
In a perfect situation with a trained tech this would be a quick repair but for someone with no experience or just starting out it would take awhile or it's time to take it to a shop. What you're hearing is when he applies pressure to the long barrel cylinder looking things, is non-rectified (AC) getting into the audio circuits via the power supply feeds.
This is the main job of the power supply capacitors, to smooth the AC being rectified by power diodes that are looking at the secondary of the power transformer.
If I remember correctly on this unit the entire circuit board would need to come out of the chasis. This is done by all knobs and nuts and lock washers on the front. Screws holding the board from the top and screws on the bottom holding the power transistors heat sink in place. Pull the board up and resolder the caps to the board. (may have some wires to disconnect also, so you would make a note where everything is connected)
Replace the mica thermal film on the power transistors. Clean and replace the heat sink compound, carefully reinstall the board. Triple check your work was done properly. (a quick check with your volt/ohm meter from the output transistor to ground should tell you everything is ok. For instance you should have a resistance reading from the metal of the transistor touching the mica film, to the chasis. How much can vary, the important thing is not to see zero ohms)
Bring up unit on a variac slowly to check your work one more time. Inject signal and watch for perfect sine wave on the scope, then test out the amp with a guitar. You can always do a power check to make sure the amp is ready to go.
And wa-la you have a happy customer!