It's the explosion of a gas station.
Once someone blew it up by shooting at it, this behaviour follows.

Same as http://feedback.dayzgame.com/view.php?id=5157

Negative When any player enters the server and when near another player makes a sound of explosion, I just checked and this new version is the same. Conform more players are entering the server the same sound are experiencing a little beside me and noticed it was the player who had just entered the server. Experimental version 47.124558 Servers

As stated before it is because the gas station was blown up. If I am going to be in a city for any length of time, I WILL blow up the gas station to use it as a doorbell. It is not for anyone joining the server. It is only characters that are within the city limits.. IE...if you are running from svet past riffy ... just as you past the nose of the ship..the gas station will blow up telling me that you are on the way. Same with the other way. It goes off just as you are leaving the middle grocery store in Berizino heading towards the Sherriff Station.

I understand because I was in town "SvetlojarsK" and kept quiet hidden in a nearly full server. When a player entered the server in this City an explosion occurs lansando smoke with fire in the roof of the "Hospital" then immediately saw the player leaving the hospital. I kept watching and more explosions when others came soon after in the same area of the hospital other players leave. Always showing the explosion on the roof of the hospital.

So what you said would be a BUG? where players can use as a signal?

Hello,

we are aware of this issue and are already working on a fix.

Please keep monitoring this issue for future updates on the situation.

Thank you for your feedback!