I just bought a 2008 GMC Acadia. I've noticed that when I turn it on and I take off right when I hit around 18 mph something thumps from the engine. It does it like 7 times all within 3 seconds, and you could feel it too. Sometimes it's quiet but sometimes it's loud. The same exact sound everytime it does it. It wont do it again unless I turn off the vehicle and start it again. Is this something normal or ???. Any ideas would help. Thanks.