Rattlesnake or Gopher snake?
Thank you for your question. Your photo doesn't show enough detail of the scale pattern on the head for me to determine the species. It looks like one of the non-venomous species. It's possible it could be the Pacific gopher snake, scientific name Pituophis catenifer.
It appears that the only native species of rattlesnake in Sonoma County is the Northern Pacific Rattlesnake, Crotalus oreganus oreganus. I can tell you that the scale pattern on the head portion of the shed skin is not a Crotalus species.
Finding a shed skin is just an indication that a snake was in the area. It doesn't necessarily indicate the snake has taken up residence. Whether or not the snake remains in the area depends on availability of prey and suitable habitat for the particular species.
I hope this information helps, and thank you for contacting Ask an Expert.