There's a tool mechanics use to pinpoint the source of a noise called a "mechanic's stethoscope". They cost anywhere from $5 to $20 for a cheap set and up to $200 for a professional quality set. I have a cheap set that I use for diagnosing problems with appliances, and it works well enough that I've never been tempted to spend the $200 on a better set.
You simply touch the metal probe to whatever you think is making the noise, and the noise you're hearing will be loudest when the metal probe is in contact with the source of the noise. You can use this tool to tell where noises are coming from.
That's because noises are nothing more than pressure waves in the air caused by movement. If you touch the metal probe to whatever is causing the noise, then the movement of that object will cause the same movement of the diaphragm in the mechanic's stethoscope, and you'll hear that same noise loud and clear through the ear phones. So, you'd touch the metal probe to the various pieces of siding on the outside of your trailer, and if that piece happens to move when you've got the metal probe on it, you'll hear that same banging or cracking sound loud and clear through the earphones. You can buy mechanic's stethoscopes anywhere they sell automotive tools.
I expect Oldog/Newtrick is probably correct in that it's coming from the vinyl siding. Vinyl siding isn't supposed to be nailed tightly down because as a building material, it has one of the largest co-efficients of thermal expansion. So, it expands and shrinks much more with temperature changes than aluminum or wood siding would. There should be "nail slots" at the top of the siding for the nails, and the siding should slide back and forth in the nail slots rather than be nailed tightly down.