There is a technique I have seen used to help break up chest congestion, where you sort of hit the person's back repeatedly. I've seen a physical therapist do this for a patient, and at some point I got permission from my son's doctor to do this for him. However, my son was older than your child. For this idea, and in general, I would encourage you to ask the doctor.
One other note, I've been told that dairy products can thicken mucous. My children's doctor encouraged us to keep the dairy products to a minimum during respiratory infections.
Edit: I looked to see if others agree with my children's doctor about a dairy connection, and apparently the jury is still out on this. I found several articles that called the dairy-mucous connection a myth; here's one that did draw some connection:
For some people, dairy products can cause mucus to thicken up, and that may cause sinus pressure and congestion. (source)
I do want to add that what you heard might be wheezing, which it might be good to have a doctor give a listen to.