Yeah, as a couple other replies said, the cracking of the sound barrier, actually the air rushing back into the gap, I think. It is a sharp crack.
The centers of each target were maybe 8-10 feet away from each other at the USMC range. They told us in the pits as target crew we’d be able to tell by the sound if it was thru (or even a near miss of) our target, apart from another nearby. I thought they were fulla... well, you know... but eventually I could tell ours apart from shots on other targets.
A silencer/suppressor on a weapon works in conjunction with ammo designed to travel slower than the speed of sound. Otherwise there’s still gonna be that *crack* but not very directionally helpful at detecting your shooter’s hide.
Hope your would-be sniper prematurely evacuates (LOL) and creates movement that draws your attention, or cycles the action or overloaded oil into the breech, creating a tiny wisp of smoke. Highly unlikely, but I could imagine the panic of someone who never contemplated, let alone practiced, the post-sniping exfil. Fun problem to think about if oh no here come the guys with the butterfly nets oh nooooooooooooo
If I was a sniper who blew my cover I’d be overjoyed if my enemies were only armed with butterfly nets.