subfloor issue after we installed hardwood floor. house < 1 yr

[Eric Mao]

I purchased a new home from a builder 9 months ago. It's still under warranty. It's a house with an unfinished basement. The main floor has builder installed engineer hardwood and upstairs has carpet.

The house has numerous issues since we moved in. The builder has already came to fix the main floor squeaky subfloor 3 times. Even today, they came and pull out of a nail under the subfloor from our basement, because it's making squeaky sound. We also had squeaky should in the stairs landing area. It was the subfloor that wasn't secured. They fixed that by pulling all the hardwood, secured the subfloor and put the new hardwood back on. All the fixes are under warranty and are relatively easy as our basement is unfinished.

We installed engineered hardwood floor 4 months ago in all the bedroom upstairs when we saw Home Depot had some floor deal. We didn't put extra screws to secure the subfloor after we pulled the carpet. After the installation, we noticed one area in our master bedroom has noticeable sound, but not too loud. Today, it's getting very bad. Just like the sound made by the nail on our main level. The difference is we've changed carpet to hardwood floor and it's upstairs, there's no access to subfloor as it is covered.

I'm pretty sure the sound is from subfloor, similar to a popping nail sound we had recently. Before I ask builder to take a look, just want to check with folks here what you think builder will do. The house is under warranty, but we've changed carpet to hardwood floor ourselves. Will builder deny the warranty because we altered the floor?

Thanks in advance for any input.