fafu

My wife and I just purchased a brand new 2010 escape limited 2 weeks ago and have since noticed that when the audio system is on the AM/FM mode, the sound quality changes. It will go from sounding normal, to sounding like the bass is being turned up with no treble, and then all treble and no bass, back to normal. We have had it into the dealership and they said "they didn't notice anything" but only drove it a few minutes down the street. They now have it again, but are fighting us that there is an issue. This only happens with the AM/FM setting. Using satellite, CD, or MP3 through SYNC sounds perfectly fine, so it doesn't appear to be the speakers. Does anybody have any idea what could be causing this? I have checked the antenna to make sure that it tight, but the sound isn't going out fuzzy like a loose antenna would. the signal strength stays strong, the sound quality just gets crappy. Thanks for any help anyone can offer.