SWings4 is the latest , and according to BQ it has the lowest noise of all.

 

You want Silent Wings 3 or 4. Shadow Wings are very quiet simply because they move very little air. The SP on them is awful, and the least bit of resistance renders them useless.

 

Which fans are quieter and more importantly which have a smoother less or no harmonic/unpleasant/rattling noise.

See QuasarZone and HWCooling's datasets and associated reviews. The Silent Wings 4 copies tested mostly outperform the Silent Wings 3 copies on a noise normalized basis. Shadow Wings 2s sometimes outperform Silent Wings 4s up to about 800 RPM. So far as I understand fan availability and pricing in the United States, there's a good selection of competitive fans at the Shadow 2, Silent 3, and Silent 4 price points which do as well or better (I think 3rsys and some of the others don't distribute much to North America, so look to Thermalright in particular).

I'm running both Shadow Wings 2 and Silent Wings 3 and the copies I got sound fine, don't show anything problematic in the spectra for the mountings I measured them on, and the operating spread I had for equivalent cooling was about 1 dB(A) in favor of the Silent Wings 3 in an airflow case at ~500 W system power. Haven't run Silent Wings 4, so no personal data on those. However, Silent Wings 4 non-Pro prices have dropped to the same as Silent Wings 3 where I am, so I don't see a reason to keep buying 3s at this point.

The Shadow Wings 2s are somewhat unique as low RPM specialists and their comparatively flat PWM curve from 450–1100 RPM can be useful in certain circumstances. I've found it's often awkward they start at 450 RPM rather than 250 or 300, though, and I wouldn't pick them for regular operation around 1000 either unless boxed into low cost selection under a be quiet! constraint.

I heard PWM can induce harmonic noise

This is incorrect. Harmonic noise derives from the blade pass frequency (or harmonics thereof) over obstacles. Doesn't matter how the fan's controlled, just what the RPM range is where the harmonic resonances are least damped.

There have been an assortment of clicking issues with some PWM controllers and some fans. From what I can tell, the industry's had those pretty well sorted for a few years now and, so far as I know, be quiet! fans weren't problematic in this regard. IIRC there were some interoperability issues with poor Silent Wings 3 PWM speed control under certain PWM controllers. Not sure if other be quiet! fans were affected, also haven't seen anything about whether be quiet! addressed the issue with a running change (the Silent Wings 3 plus AM4 ASRock and ASUS combinations I've run are fine).