De Noise “How to reduce noise in long exposure”
Shooting long exposure is not always a god choice, especially in the hotter season.
The heat make "heat noise" during a long-exposure. A longer exposure make much heat noise.
The simple solution is "not" to shoot in the hot summer. Shoot in winter, do post process in summer.
Just kidding. I'll let you know the way of reducing "heat noise". Check it out.