In order to answer your question of "How noisy is a Laing D4?", it needs to be countered with the question of what your tolerance for noise is.
Some people (you know who you are!

) think that if they can't hear something over the noise of their 40dBA PSU/CPU/northbridge/GPU/whatever fan(s), then that means that something MUST be quiet.
Some people think that the Laing D4's are quiet, while others find the whine of even the "muffled" pumps intolerably noisy. Just like how one person's definition of "super-ultra-spicy hot" food, is another person's definition of "very mild".
The Eheim 1250 is what I'd personally call a low-moderate noise pump. Not quiet at all, but generally bearable. The Laing D4's whine drills through my brain like a dentist's drill if they aren't inside an enclosure of some moderate-good amount of sound dampening material. Other people will put them in a case and say that they can't hear them.
It also depends on the level of restriction. The more restriction you put the D4's under with the cooling loop, the louder and more irritating the whine gets.