I have bought a aquacomputer D5 pump with an attached reservoir. Just got my tubing done and was doing a leak test when the pump started to make a loud rattling sound. This is my second D5 pump and the first one never made this noise. I have just the molex cable plugged in and it makes this noise. I thought it could be air in the system, but I bled it pretty good. It moves the water, but a loud way. Any thoughts?

Probably a bad pump impeller.

After running it for a while, it got quieter, but it's still there. Like a soft ringing noise. I'm getting a new D5 pump on Friday. If taken the impeller out and didn't see anything in there. No marks of rubbing or things like that. Found a very small splinter like piece, but I doubt that could have caused that much of an imbalance of rubbing. I've also noticed a slow-ish flow. I'll put my flow meter in the morning. But for only 2 GPU blocks, a CPU block, and a single 480 45mm radiator, it should not be this low for having 2 D5 pumps. Grant it, I haven't got the PWM headers plugged in to the motherboard, (both are on the Aquabus connectors on an Aquero 6 XT that has not been set up with the computer but as a stand alone unit for the dry run.) But it should be a little faster I think. I had all those blocks and 2 480 rads, a 240, and a 120 rad on just ONE pump with a flow of 89 L/PH. Any suggestions?

Glythen
I have bought a aquacomputer D5 pump with an attached reservoir. Just got my tubing done and was doing a leak test when the pump started to make a loud rattling sound. This is my second D5 pump and the first one never made this noise. I have just the molex cable plugged in and it makes this noise. I thought it could be air in the system, but I bled it pretty good. It moves the water, but a loud way. Any thoughts?

That noise can also be caused by the pump being over-tightened when connecting to the reservoir. I'd check the tightness of that first, loosen a little if it's overly snug, and try it again. 

Resurrecting this thread with suggestion and question. Since this pump has only one moving part (impeller) does anyone knows of any aftermarket impellers that are better balanced than the ones that are sold with it? That would eliminate a lot of noise and bad vibrations. 

Of course. Your point is valid. I am more into finer details of D5 operation. 
 
I have at present 3 different D5 pumps with 3 different housings and of course 3 impellers. 2 of the pumps are speed variable and one is full blast with speed sensor. Diameter of ceramic ball at the end of the shaft on each pump is super consistent (I think 0.167 in). I have tested every possible combination to make cooling setup as quiet as possible in a single case. Replacing pumps while keeping everything same produces no change in noise/vibration levels. Changing housing has impact on noise with EKWB housing being quietest. 
 
Now the impeller part. Perceived noise/vibration from one impeller is way lower than other two (they are all quiet to begin with). Upon further inspection it can be seen that all 3 impellers have balance cuts on them. It is just that this one was balanced way better than other 2.  
 
Now to your point of poor machining tolerance and fitment. You are 100% correct. If you have rattle something is way off. So now to my question. Instead of us wasting time buying new pump assemblies I was trying to find someone who sells better impellers. I have not found anyone that does this. Maybe due to fact that impellers are magnetic and is difficult to do.

No. I have no complaints. Always trying to squeeze last amount of quietness out of system. However I did test all three pumps dry for short period of time in my hand with all of the impellers. One of them created the least amount of vibration. That is the one I used. Of course headspace is important however if impeller is wobbly than you would need looser head space to reduce chance of it hitting the housing which would reduce pressure. That's where impeller's balance comes into play. Just imagine your car with out of balance tires. I was always into super quiet systems with max amount of OC. My system is (almost) completely quiet/silent when idling on desktop. So did anyone see sellers that deal with D5 pump impellers?
 
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Is it PWM version or Speed Control?
D5 pumps are from my perspective better choice.
Generally is less louder. But D5 and DDC are changed and have many variations, special DDC pump.
 
Looks like some D5 Pumps have some kind of changes on impeller and that cause to impeller need more space and cause noise.
Stay with older models...
 
These pumps have impeller like Original laing before 2009.
https://www.dazmode.com/store/shop/watercooling/pumps/pump-motors/d5-vario-8-24v-pump-wtach-cable-option-mcp655-and-etc/
 
Look customer experience on old Swiftech MCP655, 5 stars on Newegg... that can't be reach with any new PWM model from any brand I believe.
If seller still have exactly same, not to send you PWM version.
https://www.newegg.com/Product/Product.aspx?Item=N82E16835108082
 
 
That was classic experimenting on good reliable and proven product to improve his performance and to say My version is even better, than advertise agressively and give 10% more flow...
Without worry what will happen if that cause more noise  or problems because anyway enthusiasts will buy another one if don't like.
Every time when someone tried to improve German product that didn't finished good. 
On first look PWM version could look as better choice, but actually that's must worse choice, because then you met with buggy softwares from manufacturers and you don't have 5 softwares to control PWM pump, you need to use software from motherboard. With ASUS that's famous well known buggy AI Suit. If not you stuck with BIOS.
If D5 not help you on 3 speed he will not decrease temps even on 5. Speed 5 need only if you use two graphic cards and CPU and VRM cooler because flow.
Than everyone anyway use two pumps on speed 2. And they will not spin max when you restart computer, you will not need softwares, you will not need to enter in BIOS to change settings.
I change everything inside BIOS. I didn't change speed from 50% at all, that's 2400-2500 RPM. I believe she need to work little faster when I add GPU, that's exactly speed 3 on non PWM version, 3300 RPM is speed 3. And you don't need to change that and after all PWM become inferior to old one. I don't know single software to change D5 PWM except AI Suit on ROG Motherboard.
Or you can spend 50-100$ more to add controllers etc but that's complication and not necessary at all. 
 
I installed once AI Suit and after week my thought was, why people complain on this software, everything is fine.
Than one day no icons empty task bar, tomorrow I go to check CPU temps, CPU Temp 0. ****... Intel Diagnostic Fail because temp 0.
I start to look price of second hand Haswell-E, i7-5930K or similar... then I remember people talk AI Suit is in conflict with HWInfo, AIDA64, etc...
Nothing help...everything I tried or read force me to think that sensor in CPU died.
After I reinstall Windows when I saw normal CPU Temp I was so happy... never again installed AI Suit, control PWM in BIOS and never again PWM profile of pump.
Everyone should have two pumps in house, like two PSU, because if you don't have you will start PC when new one arrive.
I don't afraid of damage because BIOS will disable CPU on 80C and that's it but two pumps are necessary and I will keep in box PWM version.