I cannot find a good replacement. Then I used a 2 wire 40mm instead of the original 3 wire 25mm. Due to the wrong mounting hold pos, I glued the fan at the ventilation hole at the top instead of bottom.
I would classify the original fan into two noise level: 1. Powerup. About 20 second long duration. 2. Normal. I guess N1200 has speed control during normal opertaion. During powerup, the fan may run in full speed.
Before the original fan died, occasionally, the noise would reach case 1, very noticeable. Actually, very noisy.
Case 2 is acceptable, but noticeable. If 40mm is case 3, it is close to unnoticeable. BTW, I am using 2.5" HD internally, it is less noisy than 3.5"HD.