Mowing problem with disc mower

[big lar]

I have a new holland hm 235 disc mower and it has been trouble free for the5-6 years I ve had it, Took it out of the shed today for the new season, ran it,

There is a clicking noise coming from the disc(top-hat) closest to the tractor;

I checked the blades; they are tight.

Could it be as simple as a stone in the top hat; or worse, a bearing? It was fine when I put it away last fall