"Knocking" sounds like busted or poor quality bearings. Starting voltage at 7 V suggests poor parts.
Judging by the case and previous fans, I take it that looks matter
? Clear plastic is usually frowned upon as brittle and hence prone to unfavourable acoustics, but if you gotta have it, then Noiseblocker is where I would go. I wouldn't worry too much about putting modern sleeve bearing fans in a horizontal position, I've run a few of them just fine like that, even cheap ones like Arctic Cooling. I've heard no complaints from the people I assembled rigs for either.
If you can pull yourself away from aesthetics, the "poo brown" Noctuas with their SSO bearings have been good low-voltage operators and come with a 6 year warranty that suggests real confidence in the product. Quiet too, as evidenced by SPCR reviews, admittedly of 120 mm models. However, I have no doubt in my mind the 140 mm will match the 120 mm ones as Noctua's quality has been quite consistent. Early models had some trouble with backpressure, but that seems to have been rectified in the modern iterations.
How's that sound?
PS. Scythe, another SPCR favourite with a good track record, also has a 140 mm model based on a successful 120 mm design, http://www.scythe-usa.com/product/acc/111/sm1425sl12_detail.html
. However the sample variance at Scythe seems to be bigger than at Noctua, so I do not personally feel as confident about recommending this as I do the Noctua(s).