Hi Jasper,
I'm not really sure if this is a pump issue or fans, but if the noise is somehow dependent to your CPU temp, there is a chance that the fan is culprit. Pump rpm is constant in the H100i, nothing could change that speed because its directly connected to your SATA power. Try unplugging the fans and see if the noise would persist. If this is a fan issue, then It'll be an easier fix vs replacing the whole cooler. I'll have it replaced it via RMA.