Could a pressurized fire sprinkler system be causing a loud sound?

Every night, we hear loud cracking sounds coming from our attic.

The sounds are quite loud, and it sounds like there could be a real problem.

We hear it every night, usually multiple times. It seems to be getting more frequent and intense. It does seem to follow a very rough schedule. We often notice it around 10pm and then again around 2am or 3am.

There is no evidence of any animals in the attic.

There is nothing really up in the attic that could make a sound like this except for the pressurized fire sprinkler system. The only other possible source of the sounds I can think of is expansion/contraction of the structure. We have asked neighbors, and none of them have reported similar sounds.

Our HOA, which is responsible for the fire sprinkler system, denies that the fire sprinkler system could have anything to do with the cracking sounds. They had an a-hole lawyer send us a letter stating that we have to somehow prove it is the fire sprinkler system before they will do anything about it.

What is causing this sound? Does it represent a hazard? What do we do about it?