Hi. I am completely new to oxygen therapy, so I hope someone will be able to help me out. I just got my oxygen concentrator delivered (it is not new) and I am worried about the noise I'm hearing. I'll be contacting the company as well, but I feel like they might not be completely helpful as they've already failed to provide me with one that was satisfactory earlier. Anyway, I have never used one so I am completely clueless as to what noise levels are normal. I keep reading about a buzzing noise, which mine has, but I can't find any information on the "puffing" noise I hear every 2-3 seconds. It seems like the machine is sucking in air, makes a light "pop" and then puffs. Obviously quite difficult to describe in writing. It seems to be a "logical" noise (given its purpose) but I don't remember hearing this when I first switched it on and let it run. It also heats up quite a bit fairly quickly. Is this normal? It is a Kröber O2. Any input would be appreciated.