mooing noise when turning the engine off?

Yes, this could be the A/C system equalizing which is normal. To be sure, turn the A/C off to see if it makes the same noise. Also, can you please shoot a quick video with your phone so I can hear the noise? That way I can tell what's going on. You can upload it here with your response.

Thank you for your prompt reply. I turned off the A/C, but it still produces noise. See video attached.

Wow, that is a good one, thanks for the video. So, the HVAC system needs to be completely off because they run the compressor when the deforest and heater are on as well. When it is making the noise put your hand on the expansion block which is right there at the firewall in the video. Feels around for the source of the nose, it will vibrate your hand when you have found it.

Hi, I turned all the A/C and heating system off and still get the same mooing noise after turning engine off. I can't feel a vibration on any of the surrounding parts. Any suggestions?
Thank you

Put your hand on the EVAP tube in front of the A/C lines, it might be an EVAP purge valve. Here is the location of the valve so you can check it out. It looks like the valve is back at the fuel tank. Check out the images (below). Please let us know what happens.