Already explained that in mails but I do it again.
The D4 comes from DangerDen, please stop to compare it each time with Swiftech products as a unique design because it's not. Dan said me that it was a little different from yours (it's a muffled one he said too but I don't have both to compare them). But now, he said he will receive the same as Swiftech. Mine has a MFR of 08/2004. Here is his explanation from mail :
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All the newer pumps will be the same both ours and the Swiftech pumps, Laing was doing different things with the pumps but not any more other than trying to quiet them down a bit more.
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Nothing is resonating in the foam box because pumps are put on another piece of foam very soft to avoir all vibrations. If you had read carefully you should see that I introduced a restriction in the discharge tubing (I pinch it slowly) to go to 0 flowrate and then I relax it to go again to the "max" flowrate with almost no restriction (just a tubing). So it's normal that the noise grows up because DC pumps spin faster under high restriction (~3900tr/min), it's not a resonation sound with lower frequency... DDC is very quiet like Eheim and almost inaudible in a case with appropriate considerations !
Pay attention to the speakers volume, it must be very low to join the reality because pumps are very quiet, except D4. No dB measurements sorry, it requires good materia and technicl to capture such low sounds and I don't have any.