Help diagnosing a noise

[WestCoastSVX]

I've got a new noise, hard to describe, kind of a pulsating hum that gets faster as I open the throttle until either gets drowned out by engine noise or gets so fast it blends into normal engine noise.

At first I thought it migh even be something rubbing because i noticed it at very slow parking lot speeds. Disappeared around 25 mph.

But in diagnosing, it happens while sitting still in Neutral or Park as well, though slightly quieter for some reason than when driving slowly.

Seems to be coming from the driver's side rear area as well, though I could be wrong. When I open the rear window while driving or open the door to try to hear it from the outside it actually gets harder to hear.