ASTRO DARKS OR DARK FRAMES ARE VERY IMPORTANT, AND IT SHOULD BE TAKEN BEFORE EACH SESSION... PLUS 15 SECONDS LIGHTS WILL NOT SHOW WALKING NOISE SINCE THE STACKING WILL APPLY THE ASTRO DARK FRAMES ( BOTH OF THEM ARE 15 SECONDS OF EXPOSURE ) ANY LOWER EXPOSURE WILL RESULTS IN NOISE.
Walking/Correlated/Raining noise is the result of the DFPN (dark fixed pattern noise) of the sensor slowly moving in a consistent direction across the frame over time. This is usually caused by drift from imperfect polar alignment, but may also occur for other reasons.
The walking noise itself actually results from registering the frames on the stars. Before registration, the DFPN is in the same place every frame. After registering, if there was drift but the frames were not dithered, then the pattern "drifts", however if the drift is small enough not to actually be a problem in each frame, that means the brighter parts of the DFPN (i.e. hot pixels) "correlate" across each registered frame, producing the faint streaking patterns in the integrated stack. Hence "correlated" noise, which is the result of correlation of patterns in the integration...also called walking noise or raining noise.