Hi. I just created this account so I could share with everyone why my Lionchief Hogwarts Express was making crackling and popping sounds. I searched high and low for an answer with no luck. I hope this post is searchable for everyone else out there having the same problem!
Our set was purchased new and worked flawlessly for the first Christmas season. The next Christmas I noticed it started crackling/popping really badly, especially when moving. I disassembled the speaker (tender?) car and verified everything was tight. I verified the connection between the cars. Then last Christmas the chuffing sound stopped. I was about to throw the thing away I was so frustrated.
Finally, I disassembled the locomotive and quickly found the problem. The sound wires were stuffed behind the back left wheel. Over time, as the wheel turned, it started rubbing through the wires. This created all the popping and crackling as the wire came into contact with the metal wheel. Eventually the chuffing wire was cut all the way through which is why we lost the chuffing sound.
Anyway, I cut up an old micro-USB wire and used small sections to lengthen the speaker wires just enough to reroute them up and over the back wheel instead of the factory position right behind the wheel.
The sound works like a charm now. I love the train set almost as much as my wife and daughter but wished Lionel had considered this shortcoming when designing the thing.
Anyway, easy enough fix. I hope this post reaches everyone else looking for a solution to the same problem.