I have a 54622D with probably the same original Panasonic FBA08A12U fan, and replaced it with a Noctua NF-R8 redux-1800 ($10 Amazon). Had to file a bit of the plastic fan frame - otherwise a simple swap out, just don't use the third yellow wire. The airflow specs went down from 47CFM to 31CFM, but though OK as I'm not operating the scope in the desert at an ambient 50 degC, and the scope dates from the "over engineered" HP era. Have not experienced any cooling issues, but I do not leave it on all the time. My ears really appreciate the reduction in background noise, recommended.