If you're sure about radio freequency than I assume the preamp tubes very often can pick up even a voice or will do the noise even if you blow on them.
The tube dampers and coolers sold from Michael Percy www.percyaudio.com can work-out for you in that case. Make sure that space in preamp allowes you to "dress" tubes.
The tube dampers and coolers sold from Michael Percy www.percyaudio.com can work-out for you in that case. Make sure that space in preamp allowes you to "dress" tubes.