i agree with cubes.
the bubbling sound is quite normal and should well resemble a sound similar to the dying stages of your kettle comming to a boil when making a cup of tea, as essentually, thats whats happening.
my GTR runs a standard temp of arround 75 - 85 degrees depending on weather im driving on highway (cooler) or around town (hotter)
its good you guys are listening to odd sounds comming from your cars and keeping an eye on them, so good work for questioning it - but this one is nothing to worry about