Weird Noise Prior to Post

Before I ask I should say that I AM new to the pc building world, but I have had my machine built and running no problem for about a month now.

I started up my pc a few days ago and prior to the post beep, there was a sort of continuous beep. This is the first time since I built it that anything like this has happened. I can only assume that it's an error from the motherboard, but I don't know what it could possibly be because everything works perfectly fine otherwise.

I recently installed a 3Tb HDD (which went smoothly) and the noise started occuring after I had it installed. I thought it might be a hardware problem at first, mistaking the alarming noise for grinding, but it's definitely an electronic noise.

Aside from the 3Tb, I am running the following:
AMD FX-8350
Gigabyte FXA990 UD3
MSI GTX 660 (factory OC)
16Gb RAM
120Gb SSD
750 watt PSU

Thanks in advance for any help anybody has and if anything's not clear I will do my best to clarify. :)

I'm assuming you have a speaker plugged into the board. This is nothing to worry about as its just the BIOS running through its checks. The 3TB maybe causing the bios to stumble as it is just a large drive that may take a little longer to respond than the post would like.

I can't tell you why exactly it does it. But if the machine posts and the drive is useable, then there is nothing to worry about.