Well, at this point I would bring in another potential source: outside interference.
Can you take your SEX amp in to work to see if the same issue happens there?
What I'm thinking is that you may potentially share your power utilities with neighbors, or there may be some appliance in your home that is sending out some noise into the power lines periodically, and that may be what you're hearing.
In a lot of solid state electronics, there are regulators that can swamp out this transient garbage, so you may not have noticed it before.
-PB