Having had all 3 pumps discussed, D4,D5,ddc/MCP350(10w ver), i can only say this: If noise is your main concern, get the 10w (the older version) mcp350. D4 is a good pump, yet noisy. D5 is a great pump with good flow and head pressure, alltho no-one will ever call it silent. there is howerver a knob to turn on the D5. Goes from crap performance/low but still annoying noise, all the way up to top performance/louder annoying noise + vibrations.
mcp350 is damn near silent, but has a slightly lower flow than D5, but still good head. Sadly it comes in 3/8" rather than 1/2" connectors. THere are some aftermarket adapters, diff lid, screw-on tank etc, wich are horendeously expensive. Most ppl just get adapters, or do the mod for better frow with a 1/2" inlet, and get one adapter.
*Understand that this is just my own experience and my opinion on noise. Painstakeingly annoying for me might be ok with u. *
Also remember, ive used mostly eheim AC pumps before. They are very quiet, once u get rid of the vibrations.....
But as far as noise/performance ratio go, the mcp350 is a winner in my book.