Welcome to 2CarPros.
It's most likely not related to the service 4wd. That usually involves a sensor related issue. Is there anything that you can do which changes the sound or stops it?
The first thing that comes to mind for me is the exhaust system is loose due to a broken hanger. Make sure the vehicle is in park and the parking brake applied and craw under it to see if there is anything loose on the exhaust system.
If that all checks good, look for anything else that could cause it. Now, if you feel a vibration when there is a noise or at any other speed, it could be a universal joint on the driveshaft. Take a look through this link:
https://www.2carpros.com/articles/how-u-joints-work
Lets start with those couple things. Let me know what you find or if you have other questions.
Take care,
Joe
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Friday, October 11th, 2019 AT 6:54 PM