This may not apply to other tube types, but my 3 clocks built with 6 x B7971s were MUXd at about 2 kHz, and they all produced enough acoustic noise to be annoying in a quiet room, and RFI audible over the entire AM band that was detectable on my transistor radio to about 20 feet. The clock was enclosed on all 6 sides in a box made of 1/4" plexiglas, and the electronics (except the tubes) were in an aluminum chassis. Someone once suggested that the noise came from the switching power supply, but I used a linear supply with old-fashioned 60-Hz power transformers and the noise changed whenever the characters in the display changed. BTW, 45+ years without any tube failures.
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