I'm having same issues with a 400SC.
I ruled out the speakers by hooking up another set.
I ruled out the software, but installing another audio app.
I reinstalled the Dell audio files, but this did not correct it.
I installed another audio card, and it has the crackling, if not worse.
I'm sure it's a driver issue.
Someone please help!
Verify if the sound is still there when you unplug the power plug....
The sound is probably coming from the power brick itself and then amplified with the sound card....
Lots of people have the same issue with their power brick and dell seams to replace the bricks for free...