2004 Toyota Sienna ticking noise under the dash

Help! I have a 2004 Toyota Sienna with 66,000 miles and the color is light blue. During the last 10,000 miles, or so, I have had a “ticking” sound that seems to be coming from under my dash about where the center AC vents are. Sometimes it sounds like a large block of ice that is “cracking” or as my wife sometimes describes it; someone beating a drum with a uniform beat. Normally, this sound will commence when I start the van, although it is not a consistent thing—it may occur after I have been driving for a while. The sound will stop at random times. This sound occurs whether or not the AC is on. Normally, I drive with the AC in the on mode. I live in Southern California in Chatsworth, California. I asked one dealer service representative if they had a service bulletin which applied to this malady and I was given an abrupt “No!” (probably because I didn’t have a service record with this dealer because I use an independent mechanic). My mechanic says he hears the sound and agrees with the location of the sound, but he doesn’t have a clue what it could be. He doesn’t seem enthusiastic about pursuing the subject.