Zephyr,

I had a similar issue with my heat pump. One day out of the blue it
started making that "baseball cards in spokes" sound when it shut off.
It progressively got worse, and I found that hardly any air was coming
out of my vents, causing the heat pump to run almost continuously. I
had it checked, and it turns out that the bearings in the blower motor
were going out causing the blower to scrape against it's housing. I
know we're comparing apples(electric) and oranges(gas), but blowers are
blowers. It could be that the bearings are going out on that secondary
blower.