bump...just incase anyone has anything they could add.
very curious of the effectiveness of a a quiet ehiem pump in a realistic loop...with the swiffy block of course.

Is it normal for a 650 to make a high pitch noise ? i just added a 2nd thermochill 120.2 so i can run silent fans and the noise of the mcp 650 i added is worse than the noise of the 2 fans before.

the 4k spike, a (lamentably) known feature

EDIT
wrt the MCP650

I'm using a BI Pro 3 and an AquaXtreme pump in my main system and while it has a low hum it's not loud at all. I don't see any way it's 40 dB

derek, he is talking about the mcp650.

Bill: Is it when the 650 gets up to high rpm, or the low rpm when under heavy load that it starts to whine. I have had ppl reporting to me that it is very noisy over at XS. It seems the mcp600 is much better in this respect.

and . . . . . ?
mercy, read the threads here and on OCAU
Cathar goes into this in great detail

oy...

I wasnt asking for great detail bill. I was just curious as to wheter it was high load or low load. I personally dont much care as I will never use the pump, but its good info to have.

Sorry I should have clarified. I was responding to the comments about the MCP600 being loud; I don't find it to be a noisy pump at all. I haven't put the MCP650 into service (but I used one of the old louder D4s for a while) so I can't really comment.

Does the resistance where it begins to have a high pitched whine coincide with that where the MCP350 becomes a better choice anyway?

no, the noise is pretty much constant until very low voltage
and
just as pH described - in fact sooner

pH, looking at pq curves you must notice that the DDC would not be good for blocks like the TDX for instance that love flow. Its great for blocks like the mcw600 and 6002 but they perform much better

But two DDC's in parallel it would be a good (albiet expensive) choice.

As to the mcp600. Everyone I know that has boughten one over at XS loves them for their performance and for how quiet they are.

I've set up a few systems with a number of different Swiftech blocks, emphasis has been on quiet. If you are cooling only a CPU, don't care about overclocking (to temps just under max allowable are OK) then you'll be fine with a 6000, 1046, 3/8" tubing and a BIP.
If you'd like better performance, swap out the BIP for a BIP2 or a 6"x6" heater core with shroud.
Better than that, go with a 1048 or a HPPS (12V variant of the 1046, quieter than the 1046, seems to pump 1046 volumes at 0 head but is more like the 1048 at higher resistance).
All of this assumes that you're running very quiet fans, with airflows to match.

Also remember that you have to be careful to avoid water flow resistance (no right angles if you can avoid it, for instance) and air flow resistance (rounded cables, wires tied out of air path, non-restrictive grilles, etc.).

I've tried swapping 3/8" ID tubing for 1/2" ID (same system, wb left in place to avoid possible variation from a re-mount) and have seen, well... no temp improvement (using the crude measuring stick of MBM). Given that 1/2" tubing is going to have a larger OD, and so cause more airflow disruption, maybe 3/8" is better for low-noise. I've no way to measure this accurately, so please just regard this as "thinking out loud".