Here's an idea that has been floating in my head for the last week or so. How about a set of magnetic/adhesive attached wireless battery powered transducers that can be positioned around a vehicle. The transducers will measure the acoustic spectrum with a little microphone (mechanical vibrations with an accelerometer for extra credit?). They will communicate wirelessly to a central blind device plugged into your 12V outlet that logs all of the information.
Then, the information is downloaded to a computer via USB and the acoustic channels are analyzed. The user can choose to identify certain sounds with the software (frequencies, rattling, grinding, scraping, bumping, etc.) and the software will use the channels to locate the most probable area that this sound is coming from.
You could build just 2 transducers for your project as a proof of concept, but more would give more resolution. The application I'm thinking of would help me find where the awful grinding sound is coming from in my car while I drive fast without sticking my head out the window (which brake pad is completely worn out?) or identify a misaligned/worn tire by the sound it makes. It could also find rattling sounds inside the car too. Another form of this could exist in the car and give the driver an indicator light that there's something wrong before its bad enough that the driver can hear it.
So the project would be :
Hardware for transducers (PCB layout, mixed signal circuit design, wireless communication)
Hardware for the receiver (PCB layout, circuit design, wireless communication)
Firmware for the transmitters and receiver
Software for the PC application (GUI, USB interface, Signal processing).
Its one of those applications that probably does not have a market for the cost it would take to make it, but would still be useful enough that interviewers might be impressed.