I bow to superior knowledge [Linked Image]. Yep, the bus being the source would make sense, it's more exposed. I think case design has some bearing as well as motherboard design though, especially with today's very high speeds. One of the nastiest causes of interference I have found is bad grounding. In fact, the level of TV interference from one of my PCs alerted me to the fact that the PSU input connector's ground pin had a bad solder joint [Linked Image]
Measuring 120v ac on the case of the PC was scary.
(I traced that to being half mains voltage of 240v from the relatively high leakage filter)