I have the exact same thing going around my flock. Sneezing, coughing, rattling (not as bad now), but no discharge. I'm taking a multi-step approach because I do not want to lose any of them and I have chicks to watch over. Here's what I'm doing.
1. Duramycin (tetracycline) added to their water. 1 tsp per gallon, but I only prepare 1/2 gallon at a time so I do 1/2 tsp. This must be changed every 24 hours as it's not good after that, and you can't eat the eggs for a month once you start them on this antibiotic. Continue it for 7 days.
2. Bleach solution in their drinking water to take care of any e.coli or other bacteria in their water. The recipe for the solution is 2 cups bleach in 1 gallon of water, label it SOLUTION, and put 1 tablespoon for well birds and 2 tablespoons for sick birds per gallon in their drinking water. You cannot use a metal or rubber waterer, only plastic
3. ISOLATION. I managed to keep 4 of my birds in the coop healthy by removing the sick ones immediately. They are housed separately until treated.
4. Oxine AH. I just ordered this stuff, but it's supposed to be excellent for disinfecting and curing respiratory infections. It is safe when inactivated (you activate it with citric acid) and reduces down to simple table salt. Use a fogger or a mister above the chicken's heads so they breathe in the mist, mist their housing, the feeders/waterers, you can add some in the water as well (but do NOT mix the bleach with the oxine...one or the other!!!!)
That's what I'm doing for now. I might be purchasing the antibiotic Denagard if the antibiotic does not work on the sick chicks, but so far it's working for my two sick hens!