It can be very, very difficult and time consuming to track down the sources of electrical noise. You're at an advantage if the noise is always present, it's much easier to troubleshoot than if the problem is intermittent.
The first thing to do is figure out what circuit or circuits the upstairs receptacles are on. This may be easy if the panel is clearly labelled but I'd verify the label is accurate by shutting off the breakers and confirming the outlets are dead.
If the labelling on the panel is not clear, which it usually is not, you'll just have to have one person at the panel turning off breakers, and another person going from receptacle to receptacle. It might be a good idea to use a simple receptacle tester for this, it might find miswired receptacles, which might or might not be related to this problem.
Then the first test I'd make would be to shut off all the breakers in the house except the noisy circuit and see if there's still noise. This tells you whether the problem is from another circuit. If the noise vanishes, turn on the breakers one by one and see if one of them brings back the noise.
If the noise is there even when all the other circuits are off, the the problem is probably on the affected circuit. The next step would be to disconnect every load on that circuit - turn off every switch. For example if the problem is coming from compact fluorescent lights on the same circuits, the issue should disappear when those lights are off. New LED lights and dimmers and smart switches can make it a bit complicated to disconnect everything completely, sometimes you even have to disconnect them from the wiring.
As you turn things off and disconnect loads, see if the noise goes away. Unfortunately the only way you can be sure you've turned off everything is to have an amp clamp at the panel and see that current goes to zero.
One more thing to try, and this is a longshot - run a long extension cord down from the upstairs to the downstairs. This would test whether the problem is something in proximity to the upstairs rooms, something in the attic, something like that.