How will I know when the firetruck goes by on nintendogs?

I have Nintendogs Dachshund&Friends, and I really want to find the fireman hat so I can unlock the dalmation breed. I read that you're supposed to take your dog for a walk right after you hear the truck go by, but I'm not sure what it sounds like. Do you hear a siren, or is it just the sound of a car going by (I hear this all the time, but I haven't found the hat)? Thanks for the help!