Bruno,
It sounds like one of the tubes is microphonic, that is acting like a
microphone to produce feedback. Turn the radio on and set the volume control to
just below where the squealing starts. Then, using the eraser end of a pencil,
lightly tap each tube. The microphonic one will respond to your tapping by
producing the squealing or other noise. Replace the tube that is causing the
problem. Hope this helps you, Mike KF6KXG