As I mentioned earlier, I use one Roku box in the bedroom with earphones. The noise pops up in the loudspeakers of the TV but not in the earphones. Since quite a few days, I switched the loudspeakers in the TV off, while watching and using the earphones. No more noise! It is not a final fix, but it may help in certain cases.
Last night I bought my Roku Ultra and everything was fine, even when I plug in the ear plugs. However, this afternoon i turned on the roku and noticed there was a crackling sound. At first I thought it was certain apps, but it appeared to be all apps. So I decided to plug in the earplugs to see if there was a crackling sound, and it turned out there wasn't. Interestingly, I unplug the earplugs from the remote and the crackling sound went away. Go figure!