The dash is a piece that goes all the way from the drivers side to the passenger side of the vehicle. Could be you have change in your ashytray, could be the screws on slot above the radio are loose. Could be the glove box is loose or has stuff in it. Could be you have loose wires behind the dash. Could be the PO lost a flashlight back there and thats what you hear rattling around (oh wait you didn't say it was a rattle).
Please identify the following or there is no way to answer your question other than not t go offroading:
1) what parts of the dash make the noise (All is not a helpful answer)
2) Describe the noise, is it a clunk, rattle, or other type noise.
3) Does it happen when your going over hard bumps or is it constant
Any other information that would be helpful in locating.