Hi C.,
Is she exhibiting any other unusual behavior or impairments? If so, I'd take her to the pediatrician right away.
If not, I, personally, would just ignore it until her next check up appointment. (It may go away by then. If it hasn't mention it to her doctor before your appointment so he can observe it without her knowing or hearing you discuss it. If it gets worse before that, I'd have it checked by your pediatrician. If she will be in nursery school next year, they will probably have a speech/hearing screening there. Good luck to you.