Maybe the Impending Sense of doom means that you have not sensed the doom, but that sense will be impending.
A sense of Impending Doom, means you have sensed the doom which is yet to happen, but is impending
clear
So therefore I have a
Sence of Impending Doom
Apparently I have not been spreading enough doom and gloom about to reach departmental quotas for Prophets of Doom 