I know it's been a very long time but also hate when people leave their threads dangling and am concious of still owing a final report (:

After successfully changing the combustion fan the noise is gone. The FFBH has continued to work without problems this winter. I've not gotten around to checking for any error codes though. I also, finally, with a lot of patience successfully tested its parked heating feature and now know that it's actually working.

Removing the unit and changing the fan really is not hard. Using two small hose pinching clamps prevented huge amounts of coolant from coming out. The rest really is a dawdle since the fan is only screwed on top of the unit and changing it doesn't need opening it up beyond that.

Thanks for all the advice,
Michael