More fans with less rpm vs less fans with higher rpm

Case fans are usually the culprit but each scenario can be different. If possible use something like a paper towel tube held up to the ear and listen to various fans to try and identify which ones are making the noise, including gpu and power supply. The loudest fan will stick out even if the others are quiet and I doubt it's the nh-d14 making the noise.

Having more fans at lower rpm can be quieter. Having larger fans like 140mm vs 120mm can be quieter. Provided none of the fans are obnoxious a case designed to be 'quiet' shouldn't be necessary. They do design them to help cut noise by sometimes adding padding or by limiting how many external vents there are for sound to escape.

That can cause fans to have to be run harder to make up for the decreased airflow and defeat the purpose. Another consideration is what is the reasoning for the padded case other than to mask parts causing noise problems. It might help if the gpu fan is a bit noisy but it's sort of on the idea of noise dampening for case fans.

In theory it sounds good, in reality it's compensating for out of balance fans that vibrate or a low quality case that rattles or both. If the other parts were decent quality, noise dampening mounts for fans wouldn't be needed.

You can find some info on why cases include padding and the reality of what it can and likely won't help with here.
http://www.silentpcreview.com/article75-page3.html

The define r5 got pretty good reviews from the same site although as they went into detail the highlights of what helped keep it nice and quiet were:
Improved fans from the r4 with less rpm, less airflow, less buzzing (aka fan speed reduction)
Credit to the gtx 980 they used to test the case with since it's a semi-passive gpu which doesn't always have to have the fans running.
Removing all the hdd cages to reduce airflow obstructions.

So basically they had a nice quiet case once modified, fan speeds reduced and using quiet components like the sem-passive gtx 980. Not a whole lot of credit going to the case itself, realistically if all these steps had been taken with just about any case it too would be quiet. Not knocking the r5, it's a nice case, but also trying to be realistic. Silence is subjective and depends on other ambient noise in the room where the pc is at. For an extremely quiet pc someone would have to carefully pick and choose all their parts to use a passive psu, semi passive gpu, ssd only (no mechanical drives) etc.