Gurgling of the "chest" isn't actually from the lungs, but rather mucus, pus or blood build-up in the trachea, causing a gurgling and/or congested sound. Disease I know that can cause this are Mycoplasma Gallisepticum (MG) or Infectious Laryngotracheitis (ILT). Both of these diseases are permanent lifelong chronic respiratory diseases, that are incurable. MG is treatable with antibiotics to ease the symptoms, but it doesn't cure the infected birds and all birds remain lifetime carriers of the bacteria. As for ILT, it is caused by a virus and since it is, ILT is not treatable nor curable, it is a devastating and horrible disease to let your chickens go through. Since ILT is caused by a type of Gallid Alphaherpesvirus, it is a distant cousin of Marek's Disease, which is also caused by the Gallid Alphaherpesvirus 2. ILT being Gallid Alphaherpesvirus 1 (GaHV-1) and Marek's Disease being Gallid Alphaherpesvirus 2 (GaHV-2). The only way to know if your birds are dealign with MG or ILT, is by getting them tested. If you're in the US, I highly recommend that you talk with your state veterinarian about getting testing done for your birds. Sending out a bird for a necropsy is the most accurate way to get accurate results. Please, whatever you choose to do, DO NOT BREED from your sick birds to SELL/GIVEAWAY/GIFT, ETC, as you'll being spreading disease (which is also reportable). Keep a closed flock- no bringing in anymore birds and no letting any birds from your sick flock, leave your sick flock.
Gurgling can also be related to a crop issue if ONE bird is showing symptoms. If there are more than ONE bird showing gurgling sounds, then it's most likely a disease. Check inside their throats and make sure there are NO blockages or any obstructions causing the gurgling sound.