I started the engine on my 2003 earlier, and immediately heard a loud irregular clicking sound under the center of the dash, (sounded and felt like it was coming from behind the radio, or somewhere behind the radio)

I turned off the engine and the noise stopped, then turned on the ignition (without turning the motor on) and the noise started again without the engine running, so the engine was not the source of the noise.

It was not a regular pattern like the clicks I have heard with bad relays ( but I could be wrong about this), and almost sounded like a small broken belt, or broken fan blade flapping around randomly, so I made sure the radio and the AC/heater system were off, but the noise continued clicking away...

I also tuned the AC/heater system and radio on, and everything worked, and the noise continued clicking...

I turned the motor back on and checked under the hood and couldn't hear any noises coming from the engine, and everything seemed fine with the engine,

Then I left the motor running and looked up under the dash to try to see the source of the noise, but it suddenly stopped before I could see where it was coming from,

I then drove to run some errands, and after several stops the noise still has not returned, but I am planning to take off on a road trip tomorrow and am worried about leaving without figuring out what the problem was,

Some background on the vehicle, it is a good condition 4.6l XLT with 95k miles,

I purchased it a couple months ago from the second owner, this is the first problem I have had with it, seems to be in good condition overall, and well maintained,

Does anyone know what this noise could be?
Should I be worried about driving it tomorrow?

Any advise or ideas would be greatly appreciated, thanks!