Fix Noise in Just One Area?

There’s no sense in blurring the entire image just to fix one noisy area in your photo (that’s what noise reduction essentially does—it blurs the whole image a bit to hide the noise). Instead, get the Adjustment Brush (K) and click on the + (plus sign) button to the right of Noise Reduction (to set it to +25 and reset all the other sliders to zero), then paint over just the noisy areas in your image. That way, only the areas where the noise is most visible get blurred, rather than the whole image (by the way, if I use noise reduction at all, this is exactly how I do it—by just addressing the area or areas where noise is most visible).