2003 G35 Coupe with 135K with 5W30

So I only notice the noise while at idle. As soon as you give it even the slightest gas it's gone. It's a thut-thut-thut, but really fast. Warm or cold doesn't seem to affect it. Here's the peculiar part; I only hear it inside the cabin. Sounds like it's coming from under/behind the glovebox in the dash. If I pop the hood with the engine running you can't hear it anywhere on top or bottom of the engine, there's nothing but a nice purring engine. If you stick you head back inside the cabin it's right there, still going. There's no power loss, no fuel economy loss, runs just fine, but a little rough at idle. The only thing I could think of was maybe I'm not getting proper oil pressure at idle, but yet I'm not getting any oil pressure light. Oil is a little old, but full. Any thoughts, I need to fix my baby!