You're correct. Chickens dont get colds, they get a specific disease. Since he is a hatchery bird and only 2 weeks old, I'm wondering if there's an environmental issue involved. Some birds inhale feed granules into a nostril causing sneezing and/or head shaking. The crackling could possibly be something stuck in his throat or crop. You can give him an eyedropperful of olive oil orally, then gently massage his throat downward to his crop. He will either puke or pass whatever is stuck. A warm saline flush into his nostrils may clear out any debris, IF that is what it is.
Why would you want to give your birds an antibiotic when there is nothing wrong with them? I highly recommend that you stop giving them the antibiotic immediately. For your rooster, an eyedropperful of vegetable oil will work. Give the oil time to move/work the stuck debris if that's what it is. Then give your roo tylan (and him only) about 5 days later if there isnt any improvement.
I have used MMS successfully for years, first I did the protocol and now I use it sporadically when I start getting the flu. It is basically stabilized oxygen - since it has helped so many cases - the medicos are not really happy about that - thus controversial. But IMHO the information is invaluable. Here is the Jim Humble website:
