I am hoping someone here can help.

The other day we noticed a sewer smell in our house. I checked the crawlspace, and sure enough I could smell it. We tore a house down, put in a foundation, tapped into the existing sewer line and put a double wide manufactured home on it 5 years ago. The crawlspace floor is gravel on top of a vapor barrier. I crawled under and had my daughter run water and flush the toilets when I was laying by the connection from the house to sewer line. Well, I could hear the water coming down the pipe, checked for leaks and no leaks found. Once the water got down a ways, it bubbled up next to the sewer line going into the ground, which pushed the gravel around a bit. The sewer line runs straight to a tree which is around 50 ft. tall and 8 ft. in diameter. Would the most likely problem be roots in the sewer line?