Panaflo M1As aren't quiet by any stretch of the imagination. Try unbolting the fan and suspend it in the case while it's running. If the noise is about the same, then that's just how loud the fan is. If the noise is due to rattles and vibration though then it will be a lot quieter. I use 1.5 to 2" for all my shrouds; less and you put a pretty serious restriction on the fan's intake side (for fans pulling air through the heatercore).
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