What's the best next step for my 4 month old's nagging cough?

My daughter is 4-1/2 months old and has had nasal congestion and a nagging, dry cough for the past 6 weeks! We have been to the pediatrician's office 3 times and had her seen by different doctors in the practice - all agree nothing is out of the ordinary. "Nasal congestion is normal for formula fed babies" and "lungs are clear." 😩 The poor thing wakes up every night and has an hour long coughing fit! I'd like a second opinion, but don't know where to start. ENT? Allergist? (Is she too young for pet allergies?) Thanks!

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