Could be a timing belt tensioner bearing, alternator bearing something along those lines was my first thought.

When you accelerate how do you hear under the hood?

If it is actually coming from under the dash. It maybe a vacumm leak from a door motor or the brake booster.

If the sound fills the car when the windows are down, it may be your drive shaft support bearing below and right behind your seat.

Bearings are subject to temperature changes and with load shifts.