al most al the ALCO's have same engine machine sound.
but WDP-4's n above al the EMD's are different than ALCO's!! :-)
if its a hard chugging sound, then its an ALCO (WDM's to WDG-3A)
if its a humming loud sound as if a flight is taking off, then its EMD series (WDP-4 to WDG-4 n WDG-5) :-)