I think CKEditor is enforcing html 5 validity and noframes is not valid in html 5. You could try switching to TinyMCE editor but it may not like it either. If all else fails in the Html Content feature you can disable the wysiwyg editor and then enter raw html if you really want to. We use that sometimes to keep the wysiwyg from messing with our javascript.
But if you are targeting html 5 and using an html 5 doctype, you might want to consider whether you really want or need that noframes element.
Hope that helps,
Joe