Sometimes at the end of my exhale i can hear a whistling sound. this only happens when i exhale through my mouth, not my nose. what could this be?

Wheezing: Wheezing is a whistling sound heard on exhalation which happens when air is forced through narrowed lower airways, such as with the air trapping that occurs with wheezing/asthma/reactive airways. Treatment usually involves inhaled bronchodilators (Rx) although occasionally mucous plugs can cause transient wheezing that is self resolving with forceful cough.