You want to document your discussions, letters, phone calls, etc., etc., since you're within your 1-year warranty. Should it come to push-and-shove, it will be a he-said, she-said, as to 'when' this problem was first 'reported' to the builder. If you had an attorney do your closing, contact him for sure. Also, have your real estate agent document this too. And if you had a NEW home inspector, contact him/her too.
(As an aside, always have an attorney, always have an inspector whether or not the home is an existing one OR a NEW one. WHen spending 100's of 1000's of dollars over 15-30 years, or whatever, less than $1000 is cheap for some peace of mind and for 'future' reference.) Do get someone in there to take some samples to do some tests as Ron suggests.