My drivetrain used to be very noisey and grindy sounding. I eventually figured out that it was the chain lube I was using. It was White Lightning, the wax based stuff. I used it because it came with my Park Tools chain cleaning tool. I live in a coastal community, I ride Pacific Coast Highway and beach trails a lot, and the waxy chain lube was a magnet for sand.
I switched to an oil based lube and my bike is much more quiet, and stays quiet for much longer. Never going back to wax type lubes.