The walls on either side of the 85 freeway (as an example) in Cupertino have occasionally set up a resonance, which when the fog as come in over the Santa Cruz

The walls on either side of the 85 freeway (as an example) in Cupertino have occasionally set up a resonance, which when the fog as come in over the Santa Cruz mountains creates spots where you can stand and hear very odd "oooh-waaah-oooh-waaah" noises. The first time I heard it I hypothesized it was a didgeridoo being played nearby, but you could walk 100 yards in either direction and the sound would fade, only to pick up again another 100 yards or so further away.