What is this popping feeling/noise in my right breast? (Bilateral silicone 375cc, below the muscle. Surgery July 2017.)

Hi Heather.
This sounds like your breast implant is moving across some structure inside your breast. This could be one or two ribs or part of the capsule. It's possible that it's only your ribs that are popping. 
If you're otherwise happy it might be a good idea to leave it alone. If it really bothers you then consider an implant exchange to a textured breast implant. These move a lot less. 
You can also consider putting the breast implants above the muscle in a new pocket and away from the rib cage. You can consider an MRI to view the anatomy in the pocket. Talk to your surgeon.
Good luck. Dr. ALDO 

I am sorry to hear about the problems you are experiencing following breast augmentation.  Unfortunately, it is difficult to know for certain what is causing the popping sensation your are feeling without a proper physical examination.  It sounds like you may being feeling/hearing the implant moving along the chest wall/ribs.  If this bothers you enough to seek revision surgery, you can consider moving the implants from subpectoral (below the muscle) to subfascial (above the muscle but below the muscle lining).  This would allow the pec muscle to be deep the implant and act as a barrier between the implant and chest wall.  I would recommend consulting with a plastic surgeon well-versed in revision breast surgery to achieve the natural result you desire.  Best of luck!
Ziyad Hammoudeh, MD