This is pretty common for high ISO images. The Library view is strangely the more correct one. What is happening is that in the zoomed out view in Develop, Lightroom takes some shortcuts for more speedy display by only calculating a subset of the image for display. With very noisy images this can lead to a display that looks less noisy than reality. This is contrary to a Library preview which is done from a fully rendered jpeg copy of your image. With respect to noise, you need to zoom to 1:1 in Develop to see it correctly. At that level, you should then optimize the sharpening and noise reduction section in the detail panel. You should remember that with very noisy images, you shouldn't try to gauge noisiness or sharpness in the zoomed-out view in Develop. This is unfortunately a limitation in Lightroom that probably cannot be really fixed without making the program really slow as you can only do this correctly by calculating the entire image. This would likely make the sliders in develop really unresponsive. When you zoom in on the image, Lightroom only has to calculate the part of the image you see and so it can do a better job.