There's two common things to consider. First of all, if the battery is the original one, it has lost most of its capacity even though it may still be working. If the voltage has started dropping, some of the many computers will not go to "sleep" mode and will do weird things. GM has had a huge problem with their generators since they redesigned them for the '87 model year, and the main cause of the large number of repeat failures is a battery that is more than two years old. If your battery isn't that old, that likely is not the problem.
The second thing is there are a number of self-tests those computers run at various times, and some of them run after the engine has been off for awhile. One of those tests involves testing the fuel supply system for pinhole leaks, as in the infamous loose gas caps. You may be hearing one of those tests being run.
I don't know who Scott is. If he doesn't belong in your yard, you'd never have been made aware of the noise and you wouldn't be worried. If the noise doesn't stop and you start having other symptoms, that's the time to be concerned.
Wednesday, November 7th, 2018 AT 5:46 PM