Advice for cooling in Fractal Design S case

The noise what you hear is the air passing through the radiator and all AIOs have that additional noise issue. So, going with x62 Kraken won't help. If you want to get rid of that noise, switch your AIO to air cooler.

As far as AIOs vs air coolers go, you won't gain any cooling performance if you use AIO over air cooler since both are cooled by air.

Pros of air coolers:
less cost
less maintenance
less noise
far longer longevity
no leakage risks
doesn't take up case fan slots
additional cooling for the RAM
cools down faster after heavy heat output

Pros of AIOs:
no RAM clearance issues
no CPU clearance issues
takes longer time to heat up during heavy heat output

While how the CPU cooler looks inside the PC depends on a person. Some people prefer to see small AIO pump in the middle of their MoBo with tubing going to the rad while others prefer to see big heatsink in the middle of their MoBo.

Main difference between AIO and air cooler is that with AIO, you'll get more noise while cooling performance remains the same.
Here's also one good article for you to read where king of air coolers (Noctua NH-D15) was put against 5x high-end AIOs, including king of AIOs (NZXT x61 Kraken),
link: http://www.relaxedtech.com/reviews/noctua/nh-d15-versus-closed-loop-liquid-coolers/1

Since NH-D15 aesthetics isn't best due to the beige/brown coloring of their fans, you can go for Be Quiet! Dark Rock Pro 3 which has far better aesthetics while cooling performance difference is 1-2 degrees Celsius from NH-D15,
specs: https://www.bequiet.com/en/cpucooler/482
review: http://www.tomshardware.com/reviews/be-quiet-dark-rock-pro-3-cpu-cooler,4350.html
pcpp: https://pcpartpicker.com/product/9bFPxr/be-quiet-cpu-cooler-bk019

Personally, i'd go with air coolers every day of the week. With same cooling performance, the pros of air coolers outweigh the pros of AIOs considerably. While, for me, the 3 main pros would be:
1. Less noise.
Since i like my PC to be quiet, i can't stand the loud noise AIO makes. Also, when air gets trapped inside the AIO (some AIOs are more prone to this than others), there's additional noise coming from inside the pump.
2. Longevity.
Cheaper AIOs usually last 2-3 years and high-end ones 4-5 years before you need to replace it. While with air coolers, their life expectancy is basically unlimited. Only thing that can go bad on air cooler is the fan on it. If the fan dies, your CPU still has cooling in form of a big heatsink. Also, new 120mm or 140mm fan doesn't cost much and it's easy to replace one. While with AIOs, the main thing that usually goes bad is the pump itself. And when that happens, your CPU has no cooling whatsoever. Since you can't replace pump on an AIO, you need to buy whole new AIO to replace the old one out.
3. No leakage risks.
Since there's liquid circling inside the AIO, there is always a risk that your AIO can leak. While it's rare, it has happened. It's well known fact that liquids and electronics don't mix.

I gather that you have the radiator mounted in front. As Aeacus pointed out, this sounds like air in the line. That happened to me exactly once with my kraken x61 which is front mounted. The solution is to tip the pc on its back legs until the radiator sits higher than the pump. This allows any air caught in the tubes/pump to fully travel up to the reservoir in the radiator. You may or may not need to do this with any orientation change of the pc (like laying it down to travel/work on)

Humming to me would signify pump noise more than anything else.

Or vibration/case resonance .


I'd try unplugging the fans on the rad & run the system to see if the noise is actually from the rad/fans - the rad will disippate enough heat still on its own to make it safe for at least a few minutes of runtime

Does sound like air in the system, the asetek pumps are prone to this in all honesty & its worsened if the pump block is fitted with the tubes in any other orientation than pointing downwards towards the GPU , the extra bit of gravity feed seems to help in some way.

As the coolant warms up it expands slightly & the air is displaced so would make sense that the noise disappears when the system has been running awhile

When I tipped mine, the pump was running. I waited till the pc was idle for a while, then just lifted the front of the case off the floor. That changes the orientation of the pump and tubing and kept it there for several minutes to make sure that any air in the tubing had a chance to travel back up the tube to the reservoir. Exactly how far you'll need to tip will be determined on the tubing itself, but ideally the further back the better, almost laying the pc on its back, which would simulate a top mounted rad.

The other option is that it's not actually air in the system but an issue with the pump. Asetek makes the better of the aio pumps, Coolit (the square ones) are hit/miss for noise and the eisberg are notorious for being loud. It could be the locking ring under the bracket isn't fully engaged, could be the pump isn't tight enough to the cpu lid, case resonance or several other things including a loose screw on a fan allowing vibration or even wires hitting a fan blade. It's really hard to diagnose specific noises without exact description and location, which in many cases isn't easy for you either, so you'll have to be patient with us and forgive any wild guesses