I am able to breathe freely, but there is a whistling sound when i exhale. i don't feel congested or anything. what could be causing the whistling?

Nose or mouth: Breathing? If the whistling occurs when you are breathing though your nose it may be due to a partial obstruction in the nose due to a polyp, deviated nasal septum or an other abnormality. It would be prudent to see your doctor for an evaluation.

Whistling: The whistling sound could be a wheeze from the lung or from the trachea. Wheezing is heard when the airway is about let's say 15-85% of its size. At 80% you may not feel tight, at 15% or lower it is just to tight to even breath. It is best to see your doctor and be examined and tested for where the sound comes from.

Allergic?: More history might be helpfu. Allergy symptoms? Should probably get checked out.

Needs to be checked: You need to be checked by your doctor or a pulmonologist/allergst as you may have bronchial asthma or some other pulmonary condition you need a detailed history including history of smoking, allergies which you are being treated for.Athorough exam and possibly pulmonary function tests/chest xrays to find what you have and treat it appropriately.