My arm-chair guess: Your mic is broken.
Ok just kidding. I doubt this will be much help but I actually experienced something a bit similar with a U87 clone I built. When I first got it built and powered it up, it seemed like it worked fine and sounded great. But every time I powered it up after that, there was a fair amount of intermittent background noise, but only cardioid and figure 8. On the omni position it was perfect. Also in my case it sounded more like a sort of wind noise than the more electronic sounding noise yours is making.
Although I fixed my issue, I really couldn't tell you exactly what the cause was. I tested and/or redid several wire connections to/from the position switch with no change. Then I disconnected the wires from the capsule, reversed the capsule in the mount, changed how the capsule wires were run (and twisted the pairs) and also went over both sides of both IC boards with cleaning solvent and a brush to make sure I got as much flux residue off as possible. When I put it back together, it worked great on all pickup patterns and has been working fine since.