What is wrong with my 7 y/o Sun Conure (Heavy breath sounds / Sneezing)?

My Conure has been making congested breathing sounds for several months now, mostly at night, sometimes with sneezing and nose cleaning. We took him to an Exotic Bird Hospital and they checked his lungs, which sounded good and clear. The vet ran some standard tests, but we couldn't afford some of the more expensive tests. He doesn't have any discharge, he hasn't had any change of behavior, he seems happy, but his breathing just doesn't sound normal. He seems to use the air pockets around his eyes to help him breath, but I don't know if that's abnormal. We've been feeding him vitamins ociated with his respiratory system and it may be helping, hard to tell. Can a bird develop unusual breath sounds? Can anyone give me any advice? We love our bird and it would crush us to ignore something that could kill him. Thanks!