Have a friend to drive the car as you push on various spots on the dash once you hear the squeaking. If you push on a spot and the squeaking stops, you've located a place to inspect closer [once you are parked]. Look under the dash for any loose bolt, screws, clips, etc.
Consider this as well: When the upper motor mount looses it support, the interior of the cabin, especially around the dash and windshield rattles when the car is at a stop. Once you get moving again it goes away, but not completely. When you stop the rattle comes back.
To check the mount, (The motor mount is on the top passenger side of the engine.) While you are standing on the passenger side of the vehicle, with the hood open, have a partner to start the engine and while holding th brake, put the vehicle in drive and apply the gas in quick, very short bursts [the object is get the engine to "jump" up (revealing a bad motun mount). He/ she should not stay on the gas for more that a second.
Repeat this test with the vehicle in reverse.
If you can see obvious excessive engine movement and/or a broken motor mount (rubber seperating from the metal parts); replace the mount.