This is probably about the 4th or 5th thread about this now. Look for "clicking", "crackling", etc. It did happen after 10.8.5. It's been suggested by the Mods to reinstall Parallel Tools. Don't bother. It won't work. You'll waste about an hour or more of your life. I thought about upgrading to P9 (using P8 right now), but it looks like P9 also inherited the 10.8.5 click issues. The way I deal with it right now is I toggle the volume up/down keys until the the sound recovers back to normal (about 2 or 3 times, then crackling/clicking goes away). This is one of those, "oh wells" with Parallels. Just like the Back/Forward two finger gesture not working that was never addressed. Btw, my clickings happens on both my Win 8 and Win 7 VM's.
Kind of a side note -- if only there's a laptop out there with the same smoothness and gestures like on a Mac's trackpad, I'd jump ship right away. Lol. While I love my iPhone, I am not fully convinced about the Mac OS. Maybe Mavericks will change my mind. I mean, Windows can do quite a lot better job when it comes to heavy lifting. And Outlook for Mac? What a joke, right? Even the worthless tiles of Win 8 is better when in "classic mode". Can't wait for 8.1 either.
Here's my conclusion about all this clicking stuff -- wait for Mavericks and Win 8.1. And hopefully, Parallels will support the Back/Forward 2 finger gesture by then. Unfortunately, we'll just have to live with the clickings for now. I will eat crow if Parallels actually fixes it before Mavericks comes out. I wonder what crow tastes like. Will it click and crackle like Parallels?