Playing this sound during a phone call is said to disrupt many phone taps 
used by law enforcement agencies. Before you get all excited I’m sure this 
won’t fool our friends at No Such Agency.)

The sound is the DTMF C tone at 852 Hz and 1633 Hz.

For more details read this summary. 
Here’s Matt’s original paper (PDF).

C-tone WAV
C-tone MP3

The WAV and MP3 files are 1 minute long. 
WAV files have better sound quality compared to MP3 usually.

Of course we can drive by smell b1tches...