cp1966, this is truly some great detectve work on your part. Ground loops are the worst.

For a test you can try one of THESE. For the test do not connect the ground lug.

If the noise goes away then the safety ground is the cause of the problem. It's strange that you are having this problem even though you are using a power conditioner. Maybe the Saftey Ground (third wire) in you outlet box is floating/not connected or else something is funky in wiring of the TV?

If this doesn't solve the problem then I'd question if the outlet is wired correctly or somehow sustained damage.