Fantastic! Thanks so much!

Sorry for the confusion - it's a new battery. No corrosion evident on it or the terminals.

Another ten minutes of running and it's been doing it a bit more regularly again. I'll try sharing a video: https://streamable.com/nbtdiq

Yes, maybe "burp" is better. I just got it back out to do some testing - we haven't even put a battery in the last couple years because of the issue. (Un)fortunately I could only reproduce the burp once over a range of speeds. From what I remember, the train would burp periodically, but not at a regular interval. Frequently enough that it wouldn't make it more than a lap or two around without a burp. It would happen over a range of speeds, and I don't recall it being drawn out at lower speeds - a very consistent noise.

Perhaps it won't return but if it does, I appreciate the insight.

I have a large scale night before Christmas set with speed synchronized sound. The sound works well for the most part and it changes appropriately with train speed. Unfortunately, every so often - maybe every 10-15 seconds - it makes a distorted noise that I can't describe any better than a loud fart. Any suggestions what is happening?