This is the time of the year when the forest Service conducts their fire training. It could be that is what you saw.
do you know aircraft types? Could it have been a C-130?
That’s what I was thinking. Also, when I lived in the Valley (coincidentally I left 12 years ago) the National Guard used to fly C-130s in and out of Van Nuys all the time.
The only local aircraft used in fire-fighting are based at the old Norris AFB in San Bernardino. Thay have two of them and they are painted red and white and are 2-engined.