What is the hertz of the monitor?
If it is 2.4 ghz and you have a cordless phone that is also 2.4ghz then
thats where the noise is coming from.
I have a problem with my security cameras not giving me a good picture, I
figured out that my phone system also 2.4 was interfering with them. I
change my phone system to 5.8 ghz, problem solved.
Also,if you have wireless internet that is also 2.4ghz. You don't want to
have different item trying to operate on the same frequency..