A regular grid pattern like this is most likely a sensor/processing artifact rather than normal random noise. From the answers provided, it does not sound like classic moiré unless the subject itself had a fine repeating texture. It also doesn’t match ordinary noise very well because the pattern is too evenly spaced.

The most likely explanation is “grid banding” or fixed-pattern noise from the sensor/readout, which can become visible in darker or underexposed parts of an image and may be more noticeable at higher ISO or when shadows are lifted.

To reduce it:

• avoid underexposure, especially in low light
• use the lowest practical ISO
• give the sensor more light with a slower shutter, wider aperture, or added lighting
• avoid heavy brightening of dark areas in post

If it shows up even in well-lit, properly exposed images, it may indicate a sensor-related limitation or fault rather than user error. In short: it’s probably not moiré, but a camera sensor artifact that becomes visible when exposure is pushed or light is low.