I have a Marshall DSL50 and 1960A cab.

It's picking up a radio signal from somewhere. It freaks me out actually when it's just me in the basement haha

Is there a way to fix it? The NS-2 blocks it somewhat

I had the same problem, I thought it was coming from one of my pedals... DSL 50, but different cab.
Since I changed the cords between the pedals, no more creepy german voices coming through my amp.

Pretty weird shit, freaked me out as well
Scientists write papers and books. Engineers make money.
Usta have that happen. Coulda been the 2 million watt FM station tower only three miles away! Try shortening/lengthening the lead. Try a different power supply/octolet with surge/filter. Turn the amp to face in different direction, ala' AM radio tuning

I found it was my guitar, also. I ended up doing all listed above and the only way it would stop was to use a separate ground on the guitar! I sold it soon after and never got to finish that install, so the point was moot.
yes, as has been mentioned, this could be different things. the usual culprit seems to be guitar pups though. a separate ground for the guitar is very effective, also lining internal cavities with a thick foil helps cut out disturbances. if you are at a gig and this happens (radio, a TV remote, the bar's blender all can make unwanted noises) then sometimes changing you immediate positioning on stage will help quell unwanted noise until a more permanent solution can be reached.

overall, it is not something i really worry about.
punk isn't dead, it's always smelled that way.

"A perfection of means, and confusion of aims, seems to be our main problem."