Hi folks,
Anyone still looking for the impeller replacement guide that no longer shows up in the original post:
https://web.archive.org/web/20200929092448/https://support.canadianspacompany.com/en/support/solutions/articles/6000223884-v2-portable-control-unit-disassembly-guide-impeller-replacement
Mine started the buzzing noise that sounded like a stuck pump, and hitting it soundly around the filter would make it start working again--for a while. Eventually this cool trick obviously stopped working, but looks like replacing the impeller as shown above was exactly what it needed.
Thanks everyone for your help!
Mark