Well the noise is the 60 hz hum that the conveter that it makes to make the 60hz for the 110v.

I dont know of any filters that can help with that. However I am going to a car radio installer place today to work there. First day on the job. If I remember I'll ask them what is the best filter. Of course this filter has to after the 12 to 110 converter.

Here's a question for you. You said that you have two monitors. But only one has the noise. Are both monitors the same brand and model?

Are they in ANY way connected different? If possible can you hook up the ground of both monitors to the exact same place. Use the same bolt and stuff. How about the power supply for them, can you do the same for those?

Even if you hook them up just as a test to see if that would help any.

Rob