With my sound card, if I put my network card too close to it, I get constant static and audio aberrations (clicks, pops, buzzes, etc).
This has happened in every PC I've worked with- if the audio card's vital organs aren't shielded, then any nearby RF device is going to cause problems.
Currently, my gigabit NIC is built into my motherboard, so it doesn't cause any problems for my sound card, which seems rather well shielded. However, if my wireless PCI card is enabled, suddenly it's click pop buzz city, and I swear it sounds like a flipping bug zapper in the middle of a cloud of mosquitos. This is despite both the wireless card and the sound card having their own anti-interference mechanisms.
A few times in a few of my machines I've had this mysterious buzzing and other sound artifacts, and in most cases I just had to space out the components more from each other, or find a way to move the sound card away from the RF-noisy network card, and the problems either went away or were reduced to the point they were no longer an issue.