Some pictures might help some of the experienced people here figure it out. You aren't by chance cross gearing when you get the grinding noise are you? Far left gears on the front and far right gears on the back or vice versa can cause that. But yeah other people report towing as much as 500 pounds so the towing is unlikely to be the problem. It's possible though with your weight the extra weight from towing is just enough the flex the frame to get the effect you're seeing. Does seem odd though since the tongue weight on the trailer shouldn't be too high. I regularly pull 85-90 pounds with the trailer weight and don't get either of those issues, so I can't say what else it might be.