Rumbling Noise Behind Dash

[Mizzou]

I recently purchased a 2011 GS 350. Nice car but it makes a constant ominous sound when the engine is running. It is a low pitched rumbling that sounds like a bad bearing on a fan. The noise frequency is constant e.g. not affected by air conditioning fan speed. The noise persists for about 1 second following turning the engine off.

I equate this to finding a lump in my neck. No problem now but it CAN'T be a good thing.

Thoughts?