Something very wrong, a terrible noise, internal sound?

Hi, I’m on Windows 7, and have been using Audacity for years with no problems. I’m trying to record the audio from a YouTube video, which again, I’ve done successfully before. There is a terrible noise, even before the video starts, as if it’s recording all the internal workings of the computer as well. I’ve re-installed using the installer and it’s now 2.1.1 version. I’ve researched and have checked the recording device within my laptop and that’s now on stereo mix. I’ve got the recording set to Primary Sound capture driver. What on earth can this be and how can I fix it? (This is not just distorted sound, this is something seriously wrong!)

Got it! I’d previously multitracked myself so it was still set to software playthrough and overdub. Magic! Thank you to anyone who was about to reply!