It's most likely to be a small piece of glue from the central rib that runs down the length of the boat.
It's unlikely that the rib has actually come loose, if it had, you'd be able to hear/feel a big lump moving inside the hull.
I wouldn't be too worried about it. I had a boat for years that had a rattle in the nose.
And there's nothing you can do about it except make a large hole in the deck to see inside the hull and I wouldn't advise that!
You've already checked for soft spots - given that you didn't find any, I'd say, "keep calm and paddle on!"
Rob
Currently Epic V10 Elite, Epic V10 Double.
Previously: Swordfish S, Evo II, Carbonology Zest, Fenn Swordfish, Epic V10, Fenn Elite, Red7 Surf70 Pro, Epic V10 Sport, Genius Blu, Kayak Centre Zeplin, Fenn Mako6, Custom Kayaks ICON, Brian's Kayaks Molokai, Brian's Kayaks Wedge and several others...