That will depend on your radiator, fans, and pump. 6 GPUs in a single loop is a lot of heat to dissipate. We use a big radiator with lots of fans and a high speed pump. The pump will move the water around the entire loop in about 3 seconds flat, so the temp increase from gpu to gpu is minimal.
Ok. Just made the conversion, 237 gal is 900 L. How much noise does the pump produce?
As soon as all the air gets out of the tubes, then the pump is nearly silent. It's probably about as loud as a modern laptop hard drive spinning.