@CHILLCAT it is time for me to follow up since I've effectively solved my issue.
@lowtideride wasn't far off the mark. It seems it was indeed the fuel system although not air in the system. I diiscovered some old threads that mentioned rattles...
Reproduction rear heater motor with fan. Fits well and works great. Expected to be released in March.
This version is based on the one in my truck ('79-'80), so it's single speed as with all such motors from '74+. For other years the plug is...