What is this random clicking sound?

Hi everyone,

I’m dealing with an annoying, intermittent clicking/rattling noise coming from my PC. I recently upgraded my GPU, case, and PSU, but the sound was present before these changes, so I know it’s not related to the new parts.

System Details:

• Upgraded: GPU, Case, PSU.
• Kept: CPU, Motherboard, RAM, 1x internal HDD, 1x external HDD.
• Temperatures: The noise happens even when the system is completely idle at ~30-35°C.

Observations:

• The noise is mechanical and completely random, it can happen as frequently as every 10 minutes or as rarely as once a week, even when I'm not doing anything.
• It is not a fan/cable issue (checked thoroughly).
• Since I’ve replaced the major components, I'm stumped on what could still be causing this.

Could this be a failing internal HDD, or is there something else I should look into? Any help would be greatly appreciated!

TL;DR: I have a persistent, random clicking noise in my PC that continues even after replacing my GPU, case, and PSU. It happens randomly even at idle (30-35°C). Any ideas what could cause this?