Shiik and shook sounds while idling

My mini keeps playing these sounds every once in a while even though nothing is running (I think). Anyone know how I could go about identifying what app's causing those sounds?

They're very occasional, I'd say somewhere between 5~30 minutes. I notice them only when I don't have any music or videos playing. I thought it might've been Messenger or Chrome but the sounds continued even after I shut down those down.

Mid mini 2012 with 16Gb ram.

I'm sure it's software, it's those sounds OSX makes when there's some kind of notifications which was why I suspected Messages initially. I didn't notice it till recently because I usually have an extended speaker cord plugged in but not connected to anything else. I unplugged it this past weekend and have been hearing it since.

I'm quite certain it's not HDD noise, because I've been hearing it for the whole day today through my headphones.

I also know very well what a HDD click sounds like, though I am just new to Macs and still unfamiliar with its system sounds.

Finally solved this problem.

Sounds were caused by people logging in and out on Messenger.

Anyone else with unexplained system sounds, try checking out your messenger settings.