I first noticed this when working on website servers. My cursor on my iMac started flickering. Then the page in the File Manager started scrolling uncontrollably. It has a problem with Apple Mail which concerns just passing over the inbox opens up mails in a separate window. Some times a whole lot of mail and the noise and cursor start up. I have to close mail and try again. (noise is spoken of in a couple of lines on this post.)That was the first thing. Now, almost every time I get on the internet (all browsers) I am getting the same problem with the addition of a clickety-click sound on the Mac. ( a rapid staccato noise). If I close the browser (any of them) it goes away. If I restart all is well for a short time and then it starts again. I use the plugin Apple keyboard and Magic Mouse (blue Tooth). The iMac is i5 late 2014 27" Retina. I read that someone on here found it was a key stuck on his keyboard and that fixed his problem. I suppose that is possible but I sure can't find a stuck key. Any advice? I will call Apple if this is something you aren't sure of. I am out of warranty but they are good to me so far.