"Symmetrical noise" on H-field

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What causes this kind of noise? I'm seeing it frequently on my station and other stations as well.

What you see isn't 'noise'... Your station signal data is compression encoded, and what you see is the result at the server. If I understand correctly, those areas indicate the part of that specific channel signal that wasn't sent. because it was noisy, or otherwise not used, or necessary. You see a server 'artifact' that restores the 'timing' between the used portions.  Saves bandwidth among other things. Whether it's visible or not is controlled by the server operation...