What do they howl at?

Every now and then, the bassets will all start to howl.  I am puzzled as to the reason.  It’s not like there are wolves in the neighborhood howling, or even dogs barking.  Everything will be quiet and calm, and then all three will start howling.  Then as if it were planned, they stop, and go back to sleep.  Perhaps they are just singing a silly basset song.  Or, maybe they are plotting to take over the world, and are communicating with some other basset hounds... Who knows... But it is funny to listen to their song.