Oooer. It looks like someone has stuffed blueberries in it. There is a fungus that looks a little bit similar but not in that quantity.

Thanks. They're a bit too small to be blueberries (or likely any type of berries). Maybe I'll check out funguses.

Wondering if they could be eggs of something?

That's what I'm afraid of.

I dont know sorry ,but interesting to see what they may be,

Can you and try and get some out with a stick and have a closer look ?

I will try that when I go back to the house tomorrow. I just don't want anything hatching and coming into the house by the dozens....

Looks like some kind of scat to me. I would rule out rat. You might try pulling the vent out from the outside of your home if it is snapped into a rubber gasket. Is that a vent to a clothes dryer or stove vent?

It looks more like some kind of slime mold, to me. Is there wood and moisture in there somewhere, Melliottr?

Thanks for your input everyone. Yes, they're in the vent from the kitchen (near the stovetop) to the outside. We got a few of them out today and cut one in half. They're very light, and there doesn't seem to be anything inside. They're almost like polystyrene beads or something. Very strange.

Any children playing around the home?

I think Tug might be in the right direction. Have a look through some of these and see what you think.
https://www.google.co.uk/search?q=Metatrichia+floriformis+slime+mould

We also have these around our kitchen vents etc (and other rooms, when work opens the walls). They are the cavity wall insulation.
Try googling "grey cavity beads" for examples.

Lol - not going to hatch for a while then...

The next question is: why are they coming out in the vent now?

They do look remarkably like cavity wall insulation beads. They are put in under pressure so it would make sense that they might have found an escape crack somewhere.