If all else fails, get a passenger to hold a suitable length piece of rigid pipe with a flexy end which he can hold to his (her?) ear. Drive at the appropriate speed and let him search for the noise.
If you want to go high tech, use a laptop with a microphone (on a stick), headphones and sound recording software. The louder the sound, the closer the mike is to the source.
But try everything else first!
Last edited by 8 Valve Ed; 17th June 2015 at 20:20..