Sadly, that is very common behavior for noisy small-signal tubes in high-sensitivity slots (preamps). Especially for older tubes with bigger plates. Phantom whistling, rustling leaves, comes & goes, etc. You either have to move the tube to a lower sensitivity slot (for example, move it from an input position to the output / follower slot, or move it to a power amp if it uses the same tube type), or replace it with a more tightly screened low noise tube. New production tubes (like Russian Gold Lions) are fantastic for tricky slots because they can be easily selected for very low noise and tight matches.
The Herbie’s halos look really cool but in my experience do NOTHING for this problem. Don’t even bother. Same for isolation bases and all manner of such tweaks.