If his water cooled spindle is really louder, he bought the wrong one! Here’s a video comparing noise levels of all three types of conventional cooling (not passive). Note that a 10 dB increase in SPL is said to seem twice as loud. Electric fan (or passive) cooling offers quietness without the hassle of water. If he had decent spindle he could be running it at 24,000 RPM like he should!
Here’s what Stepcraft claims for their spindle options.