Kinda cool. I was thinking about the car noises and found a bird in the signal !!!!
So, first the new insight. 8 cylinder (yuck, but it made my mom happy, and since she was with me, very tired and in heart failure getting into trucks was a big problem for her so we got the 3rd one we looked at) so multiply the revolution frequency by 8 for the sparkplug firing sound (8×13 I lowered it cause I had a picture of it idling around 800). So 8×13 is 104Hz. Looking there you realize that the signal is split between 104 and 52 with an occasional lower frequency thrown in.
Now for the exciting stuff. I found a bird!!!! The average maximum frequency of its call is 3100 Hz, and the average minimum is 2800 Hz.
It could be a white-crowned sparrow Zonotrichia leucophrysnuttalli
(see Luther and Baptista) calling in the Lake Merced dialect. For a pic of the bird, you can visit this person’s blog: