Iwaki MD pumps have a low-volume turbine whine to them. Sitting on a table you can hear it quite well. I have my Iwaki MD30-RZ sitting inside an MDF radiator box and cannot hear it at all. If I turn everything off in the room I still can't make it out. If I then turn the pump off, I can notice the difference in the room's background noise level, but I couldn't say that I could clearly identify the pump's noise when inside the box.
So basically noisier than an Eheim, but not annoying, and if placed within an enclosure, you are unlikely to noticably hear it at all.