So, I recently took it to Volvo to have an air bag message reset due to my battery completely draining over the winter. Tech told me he found a few "ghost" errors that he cleared also. It's been 2 days and I haven't heard the sound since. Hopefully it stays that way. I usually hear the sou...

It's more like a buzzing sound exactly how it sounds in the video. But I haven't checked for vacuum leaks

It seems to be coming from the back of the engine near the passenger side.

Have you made any progress with this issue? I have a 2000 S80 T6 that makes that exact same sound.