Fan noise is very subjective. What one person may deem to be whisper quiet another will still find unacceptable.
Given my previous experiences with different laptops, such as Dell Inspiron 4000, 8500, 8600, Sony S Series, P3 line (forgot series).
I find the SZ series to be a very big improvement. Its still audible since it is always on but very quiet when lightly used. If under heavy usage, its definetly noticeable that its on but thats to be expected for it to do its job.
The main thing is that its not a whiny noise that would really annoy lot of people... but as I have said, very subjective issue.
To give an idea, my SZ when idling/light usage, makes the sound similar to like a panaflo 120mm fan at low setting, very feint noise.