If you are going to Venice and you recognize previously unheard sounds, so it's probably not because of the venetian opera La Fenice, but rather to the new acoustic flood warning system:
In December 2007 it was put into operation and provides information on the expected sea level:
• 1 Tone: 110 cm
• 2 Tones: 120 cm
• 3 tones: 130 cm
• 4 tones: 140 cm
Whenever you hear the signal, it is recommended to have a couple of rubber boots at hand. You can get yourself a pair in many kiosks (Sale Tabacchi).
On 1th of December 2008 the high tides in Venice reached a level of 1.60m which was only surpassed by the flood of 1979. Three-quarters of the city were under water. So visit Venice, as long as it still floats ;-(
A phenomenon that hopefully keeps its rarity: In February 2012, the lagoon was partially frozen. Brrrr.