My cat is howling and not sure why. he is fixed.?

I'm 31 years old and have two cats. One male, one female (both are fixed). Ever since I got the male cat as a kitten of 3months old he has howled. Now he is about 6 or 7 yrs old. I got him as a stray so exact age is unknown. Anyway, he picks a different spot at about the same time every day and just starts meowing....almost like a howl. I call him and he comes over like everything is okay and then goes back to the howling. he has been in two different apartments in 5 yrs. Why does he do that? It doesn't bother the female......they play fight and sleep together and stuff. Not sure what's going on. Any insight is appreciated.