To me, white noise is a random frequency in the audible range. Using the SOUND command gives you a specific frequency. Packing together commands to give you a randomish frequency won't be what I think of as white noise.

To make a specific sound, as the whistle on a train, or music, the chuff-chuff sound, will require you to know the frequency or key of the sound and use the sound command to approximate it.

From experience I've found feeding the output from a 12F683 into a speaker doesn't generate a very "loud" sound. You'll have to amplify it somehow and I don't know why you couldn't use a pin off the the chip to control the volume of the sound. I'm sure someone will shoot a hole in that theory but why not?

I'm thinking if you're going to go through that trouble why don't you get an actual recording of the train whistle or horn signals and put them on a SD card and just have it play into the amplifier? Might be easier and sound better than trying to approximate locomotive horn signals with a PIC.

My nickels worth.