Bronco/F-150 tranny Q???

Guys... I can't find an answer for this on my Bronco groups, so here I am...
I've taken out the 1980 Bronco (351 with C6 3-speed auto) for the coming
winter.
Anyway, If I'm driving in third and I down shift to second, the tranny (or something) clicks repeatedly for a few seconds before it downshifts. The speed of the click is proportionate to the speed (faster I am going, the faster the clicking when I downshift).
Is my C6 on the way out? Is this a tranny problem to begin with? Will it run regardless of this? The fluid is clean and okay. The truck is old but 25-year-old Ford solid!
Any assistance is appreciated!
Thanks
Brad
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