Peds Resp Trach Issues

I have a lil pt who was a 24 weeker, trached, gt with fundoplication. Now he is approx 6mo chronological and 12lbs. His trach size was changed from a 3.0 Neo to 3.5 Peds. He makes these raspy forced sounds. His lungs are clear, pulse ox 95-98 on 1/16L/min O2. If when I suction, nothing comes out. When he settles back down, his sounds are completely clear and easy flowing. He doesn't look distressed, I just cannot pinpoint what is going on. His pulmonologist has heard the sounds during visits according to mom and has not made any note of it or any changes to his care. What could this be????