Jane, Just from reading your post, I can hear the crunching like it was right here(grew up in a floor covering store). I could almost guarantee you that this sound is the plastic scrim surface of the rebond padding below the carpet. Listen to it a couple times, and think plastic bag or mattress pad; does it now sound familiar?
Some carpets have a particularly stiff backing, and this will "enhance" the new noise, but fear not; it usually won't last long. As the carpet relaxes, it will remain in better contact with the pad surface, and the crunch will be over.