That would be the beep when BIOS is ready to accept keyboard input for F2 Setup/F10 Boot Menu Prompt.
The delay between that beep and the post screen you see may be related to the hardware that is installed in your system. You may test disabling as much peripherals and options in the BIOS as possible, such as disabling boot to network and boot to removable devices. You can also test removing add in devices such as hard drives, DVD/CD drives, etc to see if this is being caused by an specific device.
Also try updating the firmware of the video card installed and try updating the motherboard's BIOS. The latets BIOS for this motherboard is available here: