Strange Sounds from My AC

My husband has worked at a local Winter Park FL air conditioning repair company for the past few years, while I stay at home with our two year old son. Lucky for me, my husband is very knowledgeable on HVAC systems and does his best to ensure that ours is always operating properly. A few weeks ago, I noticed  a strange sound coming from our air conditioner. It seemed to be working well since our home was nice and cool, but I knew I couldn't ignore the whistling sound it was making! My husband was very appreciative that I told him about the strange sound because even the smallest issue can quickly become a serious problem if left untreated! I'm glad I was home during the day to recognize the issue, since he hadn't noticed.