ok. if you say its not the sound of windows logon, then definitely the sound is from Motherboard. As you it starts as soon as the bios starts loading.
It is normal for the computer to make one or two short beeps during the startup process. However, if the computer beeps and Windows does not start properly, a more serious hardware problem exists. The most common problems that cause beeping sounds are:
Memory and heat-related failures caused by a build-up of dust in critical cooling areas
A keyboard key is stuck
A memory DIMM or hard drive cable is not seated properly
A component on the motherboard has failed
A basic computer function was unable to start
Listen to the beeping sound and write down the number of beeps before each pause, if the beeps are long or short, or if the beeps high or low in tone. Once a pattern is determined, use the following beep code table to identify which component might be causing the problem:
The following codes can vary depending on BIOS version.
Beeps
Description
1 short beep and 1 long beep
Memory problem
2 short beeps and 1 long beep (repeats 5 times)
Unable to initialize video or video card required but not installed
3 short beeps and 1 long beep
CPU configuration error or CPU type is not compatible
The following beep codes can occur while restoring, flashing or updating the BIOS:
Beeps
Description
1 short beep
Legacy floppy drive or CD/DVD drive not detected
2 short beeps
Floppy diskette or compact disc not detected
3 short beeps
Unable to start flashing (such as when missing a utility or BIOS image)
4 short beeps
Flashing failed (checksum error, corrupted image, etc.)
5 short beeps
BIOS recovery successful
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Perform the following steps to resolve beeping issues. Before starting a task, turn off the computer, and then remove the power cord. After completing a task, replace any covers, reconnect the power cord, turn on the computer, and then listen for a beep code.
Remove any media from the floppy drives, CD/DVD drives, USB ports, and memory card reader slots, and disconnect all non-essential devices (printers, scanners, external hard drives, and similar).
Disconnect the mouse and keyboard, and then restart the computer. If the beeping sound stops, repair or replace those devices.
Hot air can damage internal components! Remove any obstructions that block vents. Clean the cooling vents on the case to remove dust. If cooling fans stop spinning or make strange noises, service the computer.