I get a feeling it's magnetic based on what was mentioned. I tried the analog board in another Classic with a different hard drive (I've got a non-Apple ProDrive LPS in the Classic II).
As for what the interference is, it's just a very minor twitch of the screen when the drive is accessed sometimes. It's really not all that noticeable (only when the drive is being accessed all the time) so I think I'll just live with it. However, I will test the voltages sometime soon just to make sure (this is not high on my priority list since the board worked fine in the other Classic but I'd like to see what it is anyways just so I can rule out all possible causes--we all know how quirky this older hardware can be).
As for what the interference is, it's just a very minor twitch of the screen when the drive is accessed sometimes. It's really not all that noticeable (only when the drive is being accessed all the time) so I think I'll just live with it. However, I will test the voltages sometime soon just to make sure (this is not high on my priority list since the board worked fine in the other Classic but I'd like to see what it is anyways just so I can rule out all possible causes--we all know how quirky this older hardware can be).