My oldest, Texas, howls for no apparent reason (to me). It is largely at night. He has been neutered since 6 months, and has done it before and after I have had other cats. To me, it seems very lonesome and soulful, and I usually call out to him or go to him to comfort him. That usually stops the howling. But, it seems like he is anxious at these times...maybe another cat in the yard? He is the only cat I have ever had that does this. A little extra attention seems to be his issue.