Update on my situation: Having fixed the "popping" sound myself by tightening the screws that hold the bracket/brace between the strut towers (see my prior post with picture), I was left with a metallic-sounding clunk/thud at low speeds when coming to a stop and sometimes when the wheels were turned. I brought it to my closest dealership earlier this week for an oil change & tire rotation and asked that they look into this noise/thud I've been hearing and feeling. I left detailed instructions of how to recreate the sound with the service rep. Of course, they call me later that morning and tell me that they can't replicate the issue (bang).
I go down to the dealership that afternoon and drive around the parking lot with a technician and we were able to recreate the sound and he actually heard/acknowledged it! (yippy) Now, I must say that the circumstances to recreate the noise were different than what I had experienced in the past. It would usually happen after backing/turning out of my driveway (down a slight incline), driving down the block approaching a stop sign, and then lightly applying the brakes. As soon as I touched the brakes, and I could have sworn sometimes when just coasting and the car downshifted from 3-2 or 2-1, I would hear/feel this metallic clunk that sounded like it was coming from the front of the car and underneath the back of the engine. In my mind I was convinced it was something to do with the transmission or drivetrain.
But in the parking lot with the technician I was able to get it to happen when pulling into or backing out of a parking spot, on relatively flat ground, with the steering wheel nearly turned all the way in either direction, and gently applying the brakes. Even moving just a few feet with the steering wheel turned and then applying the brakes would cause it to happen.
Well, the technician took the car back and he started looking into it right away. An hour later I got a call that the car was all set - the technician took apart the front brakes, cleaned, and reassembled everything. Apparently at least one of the brake pads was not seated correctly or perhaps it was something with the caliper movement - the technician was not positive. But whatever it was, the technician believed that something was essentially causing the front brakes to "stick" or get caught up and the noise/clunk was the brakes freeing themselves up. When I first heard this explanation I did not have high hopes that the situation was actually fixed.
But low and behold it's been a couple of days and the noise/clunk has not come back even once! The brakes feel like they grab a little better too (not sure if this is just from cleaning or if there really was something restricting the caliper/pad movement). I'm very happy with the service I received from the dealership and am glad they actually were able to hear/acknowledge the noise and actually fix it. I am self-admittedly very nit picky and often when I bring my cars in for something like this I'm told that they can't hear anything or that it's normal operation.
Anyway, I know this is a long post but hopefully someone finds it helpful! (thumb)
Oh, one more thing - when I was driving with the technician I mentioned to him about the "popping" noise coming from the brace between the two strut towers and he said he's had two CX-9s with that exact issue there this month. Officially, there was no known issue/bulletin from Mazda and so it took him over a week to finally find those loose bolts. Apparently they are coming directly from the factory this way: bolts that are only finger-tight, if that, and end up loosening more as the car is driven. He said one of the CX-9s he just had in was only a couple of weeks old. So this is still a problem on brand new 2018 models and has been for a while, seeing as how mine is a 2017.