Motherboard maybe

At boot up system will power up but makes long steady beeping sound maybe for like a minute but not the same as what you would get with memory will stop for like 15 sec.then starts beeping a gen was fine up and running and just shut off.nothing was ever done to this computer
1 year old HP Intel Pentium Dual core
replaced power supply
no change
replaced memory no change
added a video card no change
no signs of any problems on the board
Note there's no beep code that matches my problem it starts beeping as soon as you push the power button for 15 sec then stops for like 8 10 sec then repeats