I think it’s normal to hear a bit of breathing, but whistling, I’m not sure about that… does she have any discharge or does it sound like she’s having difficulty getting air in and out?
My buns don’t go outside, but if they did I would NOT use harnesses. I’ve heard too many horror stories. Bunnies can “spook” and get tangled up in them, getting loose or even breaking a leg or their back. Some bunnies will not budge with the harness on which defeats the purpose. And buns can easily slip out of a harness. Your best bet would be to get an xpen and allow her to have that small area to run around in, untethered. You need to make sure that the space is free from pesticides/fertilizers. Also, don’t leave her unattended, a determined dog or cat could get into an xpen in seconds, not to mention a bird swooping down.
This is one of those things that works for some people but not for others and there will be differing thoughts on the topic.