I was having the same issue with a couple of my battery operated radios and it turned out that those portable charging bricks that you can use to quickly recharge a cell phone or MP3 player (such as an iPod or even wireless earbuds or headphones) was putting out some interference which is interesting because they are strictly battery powered and shouldn't have to rely on a SMPS, but apparently they do (or at least something similar) so then I just try and keep those away from my radios.
Another weird thing about those portable charging bricks is that they seem to put out interference regardless of whether they are plugged in and charging and or charging something or not..
