That was locking arm. The one that made from I believe it's plastic that lock your write read head arm.
The good news is your platter might be still working or in good health. The bad news is that paticular mechanism maybe start to loose (actually it wouldn't affect your hdd performance, I had one hdd that sound like that and it's still kicking for 5 years now).
The click sound made when the locking arm is hit the hdd case.
Overall I wouldn't use sshd for now, I had 2 died sshd sitting in my desk now. It's better if you go full hdd or full ssd.
For now I believe that sshd is prone to break faster than ssd, even when you not doing anything to them.
Dunno if they fix this problen in future.