I've heard nothing about coil whine on the Super Flower or the Kingwin on the other hand.
While i havent tested directly anything from Kingwin, it also isnt coil/whine free, i read some user reviews that point into it, for example, Newegg user reviews of Kingiwn 550W
Pros: None so far
Cons: I was hoping this wouldn't happen to me, but my PSU is producing the high-pitched whine others have mentioned. Not 100% modular
High Pitch Whine
Pros: It...supplies power.
Cons: I purchased this exorbitantly priced PSU precisely because I wanted to avoid the high pitched coil whine. My unit is VERY audible.
My trust was misplaced.
From Amazon user reviews of the KingWin Stryker 500-Watt ATX 500 Fanless Power Supply STR-500
This power supply works great, except for a very high-pitched whine some people can hear. Now a word of caution before you switch to any fanless power supply. Consider what it will do to the airflow inside your case. You'll be taking away a major source of cooling, replacing it with additional heat which will somehow need to be removed. (Note the big heat sink on the side of this power supply. Where do you suppose that heat is going to go? Yup, into your case.) That's a double-whammy many people don't consider. After I installed this unit, the temperature inside my case went up considerably, necessitating adding another fan. Now my computer is even noisier than before! So what was the point of my buying a fanless power supply???
Since KingWin sells an almost identical unit with a virtually silent fan, I recommend considering getting that instead. It's quite a bit cheaper, and will keep your system significantly cooler. As a computer geek with gobs of experience, I can tell you the biggest threat to your system is heat. Want your system's components to give you years of trouble-free service? KEEP THEM COOL!!!
P.S. If you have one of the newer CPUs and motherboards in your system, this power supply may work just fine. Unfortunately, I built my own HTPC/DVR way back in 2006. Its now ancient Pentium D 805 CPU alone can easily generate 95 watts. Add that to the old front-side bus on the motherboard and memory modules with their own cooling fins, and I have a virtual toaster-oven inside my computer's case! So for me at least, a fanless power supply wasn't such a good idea..
I still think Kingwin/Superflower is a better choice, but at the same time im not sure its totally Seasonic fault, i feel its more a combo of hardware that makes the PSU whine/coil. I own a X-660 and X-400 and i dont have any coil whine