If you had prefinish installed then your warranty could be jeopardy. The NWFA has standards. Not guidelines. They are there for safety and for consistency.

The manufactures instruction for install is for optimum results that's going to give you the best result. A professional installer will know these and they don't vary from NWFA standards. Some minor details could be different like the particular style of trowel notching for glue downs for instance

But in your case it sounds like you have more of a structural issue. While you're builder is probably correct that your subfloor is safe to walk on it sounds like it wasn't built to the standard the floor manufacture recommends to have the best result. And by installing it without following the recommendations then you're warranty is voided and thus you can't go after the mnanufacture if the product fails.

Its good you got it inspected. What did the inspector say?

A certified inspector's testimony and report holds weight in court, as it is considered expert testimony.

I agree that the flooring manufacturer would most likely be off the hook, if the manufacturer's guidelines for installation were not followed.

Get a good audio video of someone walking on the floor.

Inspector said it was not installed correctly as per the manufacturer's instructions. We keep trying to get a good video, but can't seem to quite capture the severity of the noises. We'll keep trying, though. Thanks for the replies!!! I think we have to start talking to a lawyer.