I recently bought an air compressor for airbrushing and other uses. I'm not familiar with air compressors so I was wondering if it was normal for the air tank to have sand/gravel inside it? When I tip it over from side to side, I can hear something like sand/gravel moving around inside. The manual doesn't say anything about it.
No, there should be nothing loose inside there by design. If it is a used compressor, then I'd expect it is some rust that has broken loose. If it is new, then it is probably a piece of scale from when the tank was welded, or some other junk that got left in the tank. If it was a cheap compressor, then you got what you paid for.
I encountered the same thing on my recently bought air compressor. I decided to shake them out while changing the drain plug with a ball valve (I always do this before first use, because drain plugs have a tendency to clog up).
I removed the drain plug and poured some out, they looked like weld spatter so I figured they must be left over from the welding process. Only later I realized it was not weld spatter (because there was just too much of it to be spatter) but it was the granular flux used in submerged arc welding.
I decided to keep them inside the tank because this granular flux actually contains sacrificial metal particles, which oxidize before the base metal. This effectively prevents (or at least delays) the rusting of base metal.
If you have to keep the drain plug for some reason, consider shaking the particles out because of the increased clog up risk, but it is much easier to change the plug with a ball valve.