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.