Newbie and a question!

Hey all! I hail from western Mass. How 'bout you?

I have a question for veteran chicken enthusiasts. . .
About 3 or 4 weeks ago, I purchased our first chicks from Tractor Supply. All has gone well with them - they're growing like weeds - but I've noticed that they've begun sneezing and a couple of them have occasionally had a "bubbly" nostril. They've been kept indoors and only have access to one another right now. They eat voraciously, seem extremely active and friendly. I've thought I've heard a cough now and again, but I can't tell if it's a cough or the beginning of a cluck. Lol. Their eyes are clear, no redness, no discharge and virtually no discharge from the nostrils except the instance of "bubbly nostril" once or twice.
In all regards they seem healthy and they're growing their adult feathers.
Do chickens get "colds"? What's the best way to treat this? Although they're not listless, I've just given them a mash with probiotics in it and they're getting tetracycline in their water.
Any ideas on what might be going on with these guys? Is this something I ought to be worrying about?

Thanks!