High pitched intermittent highway sound

I'm hearing this sporadic high pitched sound. It last about a second. About 20-30 seconds between beeps. I think it's coming from the back of the cab. Girlfriend thinks it's coming from the radio dash, but radio is off. I heard it for the first time today when driving down the highway. It's a Tacoma 2005 TRD sport. Any ideas? Thanks!

 

try taping over all the seams on windows, door, fenders, etc. Keep going until you find it. Also, when you do hear it - try cracking your drivers window and see if it goes away.