Sometimes vintage tubes, even current production ones, will develop oxidation when they sit unused for a long time, which causes a crackling or sizzling sound. If you use some very fine sandpaper or emery cloth and clean the pins gently that usually gets rid of the oxidation, and hence the noise. Other times, vintage tubes may develop micro-phonics or noise from age and there's nothing you can do about it. So try cleaning the pins to see if the crackling sound go away.