The 4Hz continuous noise

I noticed that there is always unknown continuous noise near 4hz (roughly between 3.3-4.7hz), sometimes it becomes stronger (even reaching 2000 counts and 1.2 STA/LTA ratio).

The picture below shows it becomes stronger.

The picture below shows the noise at 4 o’clock(UTC+8) in the morning.

Does anyone know what causes it, my station is RED68.

I read somewhere that this could be idle tones of the digitizer or the natural 4.5 KHz resonance of the geophone, but just a guess.

It seems that there are varying degrees of noise around 4hz, 27hz, 36hz, and 42hz.