I have a 2010 Grand Vitara 2wd 4 cylinder auto with about 18500 miles on it. For the past couple of months I notice a rumbling sound when I get up to about 45mph then it goes away. When I start up from a stop it sounds normal until I get to just about 45mph, than when it gets to about 50mph it goes away again. I know it's coming from the engine just from how it sounds. It's a kind of rumbly-grumbly sound that kind of slightly vibrates through the car. Any ideas???? Thanks!