As machines age or more specifically the Electrolytic Capacitors (average life span of 5 years) you get more noise seeping into the audio areas. It's a bit of a nuisance and if the sound cards can't cure it I move to some USB SOUND CARD before replacing the motherboard.
Here's one under 3 bucks to your mailbox in the USA. http://www.amazon.com/Virtual-5-1-surround-External-Sound-Card/dp/B000N35A0Y/
Hi, my desktop speakers produce a background scratch/static when the my computer
is processing(?) and even more noticeably when I drag the mouse pointer.
I have replaced the speakers so I know that is not the problem. My
sound source is on-board. I bought a new sound card but could not find an
available slot to install it. Help?