Several beeps of an active sonar. This is a low frequency sonar (long range but low definition). It is a triangular wave of 1030 Hz frequency easy to put in loop. In reality, a military submarine usually has passive sonar, more sonar info: en.wikipedia.org/wiki/Sonar. UCS Category: BOATSub. Duration: 00:31.
Civil siren of the "national alert signal" if the alert is over. 1 cycle of 30 seconds.
This is a realistic montage made using this imperfect sound, so feel free to do the same to make your siren sound like what you have in mind: bigsoundbank.com/national-alert-signal-remalard-61-s3257.html. UCS Category: ALRMSirn. Duration: 01:18.