Horse with possible allergies, heaves, treatment?

My horse is a 4 1/3 TB gelding. I got him about a year ago and ever since I got him he does this strange breathing sound. It's not coughing it's more like hard breathing. Especially at the canter and run his breathing is very loud. Sometimes the noise and severity fluctuate with how excited he is. Once I stop he isnt gasping for breath or breathing hard, it almost reminds me of asthma. He is a little sweaty after a ride but not too bad I dont think anything out of the ordinary of a workout. The experienced rider who leases him doesnt think its heaves because he doesnt have the "line" that can be ociated. Her horse that she retired had heaves. He has a very well ventilated stall, no runny nose discharge. His eyes do run sometimes even when he wears a fly mask. I have been researching Vitamin C. What do people think about it for my horse I know there has been no know toxicity reported but have read it would help with asthma like symptoms. Yes I know I need the vet but Im trying to avoid a scope and all other tests. I dont want to jump to conclusions. Note: He is a healthy horse no weight loss no breathing problems coughing nothing like that only under saddle.