After the PC is at rest for a few hours, I press the power button on the tower and a motor sound kicks in about a second or 2 near the BIOS screen (where I can enter BIOS) and it stays constant throughout the PC operation. As a temporary fix, I found out that if I shut it down and press the power button again (sometimes 2-3 times), it stops making the noise. The noise is concerning but after the restarts, it seems to operate normally.
I've had a look inside the tower and there's nothing blocking the fans, which was my first thought. The noise seems to come from the front of the tower where the hard drives and the CD drive are. There's nothing inside the CD drive.