So Cal bird makes knocking sound and live in pines??

by Tom K
(Hollywood Hills, LA)

I can't see it but it's always in large pine tree, I think a redwood. Usually heard it in the morning and late afternoons. It make a series of knocking sounds. Maybe 10 knocks as if it was hitting a hollow wood drum. It's not a woodpecker. The sounds are the same tempo and pace each time. What is the bird?