I have the DDC-2 (18W) with Petra's top, and there is no noise that I can hear. Stepping up from a D4, it was a drastic improvement in both noise level and flow.
I have a very quiet system, only one fan in the case (antec tri-cool, lowest setting), and I can not hear the pump over that.
i have had a D5 and a 18w DCC with the AC modded top and my vote goes for the DCC, however it does make noise.... it is not silent, they bot hare louder than yateloons @7v