Not sure if it also affects the other two locos on Sandpatch but the bell on the SD40 will every so often go from a bell sound to a weird squeaky / squawking sound before then returning to normal.
Took notice of this as well, on the AC44CW. The bell sounds normal for a bit and then, proceeds to randomly make a bird-like chirping noise? (Best way I can describe it) before returning to normal. It does this in a random sequence too, after having the bell active for a period of time.
