Hardwood floors can squeak for a few reasons. If you have access to the floor joists from below, you or your contractor will have more options in resolving it. I've heard about some people using a lubricant such as powder graphite in the cracks as a temporary solution, since that's where a lot of the squeaks come from.

Did you do anything to secure the first layer of plywood onto the floor joist? There could be movement there. Also the joist supports may be insufficient to be 'squeak-free' and need additional bridging.

As to why the squeaking just appeared... perhaps the extra weight of furniture has a role in things, like if it has created additional sagging/movement. Another guess might be that it's related to how wood expands/contracts in relationship to local climate.

Feel free to contact your installer and see what he can do. However, it's common for wood floors to squeak and they squeak for many reasons beyond faulty product/installation so I'm not sure how much of it will be covered under warranty.

Could also be a cleat or staple from the installation is rubbing an other nail or has punctured a duct.

coeng, how have your somerset hardwood floors held up? We are considering them.