So, I have a '97 Altima that has been making a rather interesting noise over the past few weeks, and I'm curious about others' opinions..

I think I can best describe it is a 'rotating, grinding, somewhat bubbly noise'. It's much more audible from inside the cabin than outside, and seems to be coming from the center (maybe closer to the passenger side) of the engine closest to the cabin.
It's significantly quieter in park and neutral, and louder when in gear. However, when idling in P or N, it will suddenly become extremely loud for about a 15-30 second period, and then disappear.
It's an automatic transmission.
The sound also seems to go away between gears.

It doesn't seem to have the traditional squeek or chrip that a bad pully/belt often gives, but definitely sounds like something rotating. It changes pitch.

The reason I'm asking this question, is because I don't yet fully understand what goes on in the depths and bowls of my little car. Maybe someone who does has some idea of what it could be.

Here is a video with the sound, you may have to turn up your volume, as my phone isn't the best of quality for recording. I am interested in opinions and thoughts.

[youtube]http://www.youtube.com/watch?v=SRNcwB3pQms[/youtube]

Thanks in advance!

anyone?