I flew my S107 almost constantly for a few months, and I burned out the motors. They would still spin the blades, but not fast enough to get off the ground. I found a local hobby shop that sold me replacement motors for $3 apiece (there are two and they're not the same). After another couple months I burned out the replacement motors too. I think I just flew it too much! Still worth every penny though.
I know this is an old question, but it may help somebody in the future. I agree with Gordon. Sounds like it's just been flown a lot and is ready for a refresh. Completely normal and even if these had warranties, I doubt it would be covered as the motors would be a wear-n-tear item on these little guys. Happy flying!