Just registered to share my findings related to this topic. I'll just quote what I posted on another forum first-off:
I was having the exact same thing happening to me running Win7 x64. I had Ultimate installed (work license), and I started noticing it a lot. Decided it was time to reformat, maybe it would resolve the issue... no dice. Switched to Professional (school license) rather than Ultimate -- nothing. Tried every combination of NIC drivers I could (dual onboard gigabit).
Bought a new sound card (Asus Xonar DS), still did the same thing. Disabled all the networking stuff I could while retaining usability, nothing. Dropped my OC and went to stock speeds, no good. Tried every combination of BIOS power saving, still nothing.
Decided to buy a 3.5mm-->RCA to replace the toslink cable(s) I had been using, just to see if it was an issue with the digital audio in Win7... nope, analog still crackled.
Was linked to this thread: uninstalled MSE and switched my active library to a different drive than my active uTorrent (I keep nightly backups on separate drives, so no big deal)... IT WORKS!...
I think... the sound was crackling like mad right before I made the changes, and without rebooting, the crackling is gone.
In the thread I posted this, it was suggested that uTorrent was frequently reading from the same disk as my music was stashed on, causing stuttering. Another idea was that MSE (Microsoft Security Essentials) was the root of all of the sound issues.
Since I posted that, I have switched my foobar library (music) back to the same drive as my uTorrent seeds and there has been no crackling. I do not have MSE installed anymore, though, and there is still no crackle.
I don't know if it has been suggested here yet, but for anyone else having trouble, try disabling/uninstalling your virus scanner and see if it resolves the issue. It worked for me, maybe it will work for you!