However when sitting at the desk, it is still too loud. Particularly the supplied fans of the case are producing some strange clicking sound which is kinda annoying.
Are you connecting the fans directly to the motherboard headers? if so what headers? SYS_FAN? if so go in the bios and into MONITORING, go into SYS_FAN and check they are at 50%.
Have you tried the supplied fan controlled on the case? if you set it low it should go very close to 400rpms, while i dont use it could be a pretty cheap way to lower the noise, but personally i like dynamic control by the motherboard, this is why i dont use it.
The board has 4pin fan connectors, what alternatives fans would you advice? 120 or 140? would you increase from 3 fans to 4?
First you need to understand how MSI works, the gaming 7 is the new version of the GD as far as i can tell, so it should be the same as mine, the board has two type of 4pins,
1) Voltage controlled Headers
SYS_FAN1, SYS_FAN2 and SYS_FAN3 are 4pin physically but they will be voltage controlled, they have higher restrictions, in mine is 50% the least they can drop off.
2) PWM fan headers
CPU_FAN1 and CPU_FAN2 are true PWM fan headers, in this headers you can only use 4pin PWM fans, like you are using atm with the NH-U14S that comes with a NF-A15 PWM, this type of headers can drop down to 12.5% PWM and increment in 12.5%, so you can set it up on the bios from 12.5, 25, 37.5, 50, 62.5, 75, 87.5 and 100%.
what alternatives fans would you advice? 120 or 140?
There are more options on 120s, but at the same time i prefer to take the advantage of the 140mm fans support the R4 and XL have, if you wish to read some check First 140 mm Fan Roundup: Noctua, Phanteks, Xigmatek
and Second 140 mm Fan Roundup: Antec, bequiet!, Corsair, Scythe
, in there you will find that Phantek and Antec are among the best options for 140s.
But you do need to decide how you will control them.
1) Voltage controlled, i would pick Antec True Quiet 140, they are cheap and have some of the best sonic signature of 140s, they are quiet fans, but they also dont spin that high. I leave you a screenshot of the range they can work out, its tricky to get them 200rpms though with an MSI motherboard, on CA_steve build
you can see he manages to takes them to 400rpm using the SYS_FAN header at 50% on the bios.
2) PWM controlled, here there are very few options, specially that can drop that low, my suggestion is Noctua NF-A14 PWM, their sonic signature is not as good as Antecs, but they will drop super low on an MSI 4pin PWM fan header on pure bios, CPU_FAN1 or CPU_FAN2, this is what i did, you just need to get a a PWM fan splitter like GELID PWM Splitter Cable CA-PWM-03
or Swiftech 8-Way PWM Cable Splitter
, connect them to the CPU_FAN2 and the fans to the splitter and then go into the bios and set up either 12.5% or 25% as the minimum PWM and they should drop between 225rpms to 400rpms. Just a warning the fans are quiet below 650rpms, but higher than that starts to get noticeble, higher than 900rpms they are loud, but i also restricted them so they cant go that high on the bios.
would you increase from 3 fans to 4?
Yes, i would go with 4, the two fronts and bottom as in and the back one as out. You can click on signature desktop for a very similar build with 4 fans on R4 which is almost the same as you XL.
If you need help setting up the bios, let me know.