Did my pump go bad? (Swiftech MCP50x)

My pump seemed to be working fine (there was some audible vibration) during normal use. But randomly it started running at a "slower" speed. I can still feel the thing vibrating, but noticeably slower - with a faint rattling sound. I can't get it to run at a higher/full speed despite changing the PWM settings in the BIOS (mobo: Gigabyte z170 UD5)

Funny thing is, every time I boot up the PC, the pump starts up at full speed, then "slows" down just a little before the windows log-in (about 10-15 seconds in). This leads me to believe the pump is fine, but I'm a little confused as to how to control the speed.

The temps still seem fine, even under load, peaking at 39C - in fact virtually zero temperature difference than when it ran at "high" speed. So with that it's hard to say that the pump has gone "bad", but the random change in speed is puzzling to me, so if anyone has any insight as to what is happening that would be great. It's been like this for 5 days now.