Sounds like the sponge they use to seal the vents, it degrades after a few years. It actually makes very little diference whether it's there or not anyway.
Run the system on high for a few minutes every week just to get the loose stuff out.
Nice old perished foam, becomes all gooey and messy!! So be careful when the pieces come out of the vent - gently collect it and throw away, otherwise if squashed and smeared it can make a nasty mess!