It's really hard to say since we cannot hear it.
Since you've gotten two of these units so far...and noticed the "clicking" on both of them. It would seem to me that either you are the most unlucky person in the world to have gotten two MacBook Pro's making this "weird" clicking sound...or you are being hyper-sensitive to some sort of noise that is totally normal for a MacBook Pro.
FYI...the hard drive in the 13" MacBook Pro's from at least the last three models (2010-2012) is located right next to the CD/DVD drive...so I'm betting that this "weird" clicking that you're hearing is simply the read/write heads moving back & forth as they access (read/write) to the hard drive. Different hard drives make different noises & sometimes the hard drive sounds can be louder & softer (even two hard drives from the exact same manufacturer & capacity).
So I'm betting the "clicking" you're hearing is the hard drive just "doing it's thing"!
- Nick