RE: "I did 8x sampling..."
I have found multiple sampling to be somewhat effecive in reducing noise on underexposed areas of film. The improvement at 2x was noticeable. At 3x there was some additional improvement that was discernable; I did not see anything at 4x to justify the extra time.
However, what you are showing here looks a lot different from what I call noise. When I see noise on underexposed film, the pattern is a random distribution of faintly colored dots. I agree with others who suggest your regular, moire-like pattern seen on the slides looks like it might be some kind of electrical interference.
While it is probably not the cause of your problem, I have noticed different sharpening routines can magnify some undesirable scanning artifacts. If you have good post processing software, my advice would be to not let your scanning software do any sharpening. Chances are, you will have more control and get better results when sharpening with something like Photoshop.