First sound an iMac 7.1 makes?

I recently had a HD failure and I replaced it with a 256 gb Samsung 830 SSD.

Everything is working fine except I sometimes hear this mechanical sound "eeeh-iiiih"

Its the same sound that I hear during startup right before the mac chime, the only problem now is that it appears randombly, even when I am away from the computer (since its acting as a plex server)

Is that the sound from my harddisk or optical drive, I tried inserting a disc to "reset" it which was one of the suggestions I found online.

Any other clues that could help me find out what noise it is?

Ok, makes sense

Anyone know how to make it stop?

I am going home to try the harddisk without sleep mode to see if its help since I guess its related to waking up from sleep...