What have I done?
The bell starts when I use the horn,
and goes off with the NeXT use of the horn.
Using legacy for control.

Probably nothing. I just picked up the Dash 9 set, and the engine does funny things every now and then. The horn sometimes goes off by itself while the train is running around. I started the bell running continuously, and it wouldn't stop when I held the bell slider in the up position like you are supposed to.

The stuff doesn't happen all the time, just now and then. I usually try a re-set of the loco, or a re-start of the session and then all is usually OK for a while. I have other Legacy engines that do these wierd things every now and then as well.

Try the re-set trick, or maybe even do a complete re-load of the Legacy module for that particular engine. If the problem persists, and stays consistent, meaning doing the same erratic thing all the time, you probably will need a repair.

REV

This may sound silly, but is the track polarity correct ? positive to center rail. Reversed polarity will cause the bell & whistle to function backwards.