This is a long shot, but one time when I had my car serviced they returned it and the gauges were pulsing at a constant interval and I could also hear a little bit of noise behind the dashboard. I took it back the shop and they said it was probably a bad voltage regulator. I swapped out the VR and same problem. I was scratching my head because I could not figure it out. In desperation I started messing around with all the dials on the dash and it turned out to be the wiper delay switch was on or engaged in some funky mode where all the gauges were pulsing but the wipers were not moving. Once I turned the switch to off it quit doing it. This is most likely not your issue but I figured I would throw this out there if it was something you wanted to check. Hope you get it figured out. Keep us posted on what you find out.