Cracking/rumbling sound in chest?

Hi, I have been having this cracking/rumbling sound in my chest for about 3 months now or at least that's when I noticed it. Whenever I take a deep breath I can kind of feel it but I can feel it the most when I move my stomach in and out. It feels like it's in the middle of my chest and it doesn't hurt when I do it. I'm only 17 and have been having anxiety over this. I went to the doctor 3 months ago and they said everything sounded normal. I was wondering if this could be something serious or just something small? I use to take Prilosec because the doctors thought I had acid reflux but I quit taking it because I don't think I do. I do not take anything for anxiety but might need to get on something soon before it gets out of control.