As for the general issue, I had the exact same problem, and am now on my third rMBP. The first had severe creaking and cracking sounds, and the second had similar "popping" noises. I ordered a pentalobe screwdriver and removed the bottom cover of my second computer, and found that the springs were indeed the cause of the noise. I could press gently on them and exactly replicate the sound. It was a grinding/popping/squeaking sound - it seemed as if they were not well lubricated. At any rate, I also had the issue of the bottom plate not being flush (it bulged out near the air gills on the side) and a squeaky right arrow key. Once again, the computer was returned. I am now on #3 and everythign seems fine so far. No squeaks, rattles, creaks or pops. I'm a happy customer.
For those of you experiencing this problem, I think it is a manufacturing defect with the early models, or a random hardware issue. For 99% of the population, these subtle noises would go unnoticed. And even those who did notice problably wouldn't care. But we are not the 99%. We pay premium prices for premium products, and anything that doesn't make you extremely satisfied is unsatisfactory. I encourage anyone who is not absolutely content with their computer to take it to an Apple Store. They work hard for their reputation and to make their customers happy.