There are plastic privacy slats in the cyclone fence surrounding the parking lot at work, and when it’s hot it sounds like Rice Crispies crackling. I figure it’s the heat, but then I’d expect them to expand, and the noise stop. Instead it goes on all day long. What’s happening here? It’s not the wind, as it does it on windy days and calm days.
It’s likely the heat (as you said). Does it stop later on at night, once the temperature steadies?
I know my aluminum siding makes creaking sounds when the sun is on it, then does it again around dusk. Granted, it’s only for an hour or so each time, but different materials do different things.
Dunno. I’ve gone home by then. 
If I remember I can try to check early in the morning.
If the sun is on it and/or the air has already started warming, it may sound like it does now.
You could try yanking out out (though it looks like they’ve expanded to the point where that could be difficult. Assuming it keeps going it after it’s removed, see what happens when you have it in your house for a few hours. I’d bet it stops after a while and starts back up as soon as it’s outside in the heat.
I’d also bet it doesn’t do it in winter, assuming you live somewhere with cold weather in winter.
I suspect his workplace would resent him taking slats home to play with. 