Well Solid, if the pitch and volume of the grumbling slows down or changes as you slow down, or work the brakes, then it is most likely rotational. Otherwise, maybe you're just dragging a stick. Just work your way down the list. Does it vary with engine RPM as you downshift, or is it linked to wheel speed instead and not engine speed. Do you hear it when you slowly accelerate? Front sprockets are in the news lately but they weren't a common thing on the 2012. But, certainly, I would check tightness of the front sprocket bolt ASAP and inspect. Something in your belt - something on your belt - bad rear wheel bearing - bad front wheel bearing - grossly misaligned belt - it tries to climb the flange on the rear sprocket - worn brakes - cupped / chipped front or rear tires. Do you have a belt vibration dampener that could have gone bad? To start with, ask your lovely assistant to push it across the garage floor while you get down and give it all a listen. Or perhaps you'll be able to feel the grinding or vibration. A short ride without your helmet may help you hear it better. Jack the rear and front wheels off the floor and give a spin by hand and see what you feel. Feel the tread on your tires checking for cupping / chipping. It's just going to be process of elimination for you, and I'm sure you've already checked most of these. If you decide that it is internal to the motor, well, that's a subject for a different post. Not typically the case, though. Good Luck.
