Don't be sure that it's not the spocket. Most don't replace the front sprocket with a Suzuki part. The OEM part uses rubber to deaden drivetrain sound. When I added my old 16 tooth, it was loud as it was solid steel.
Third gear may be louder than the others due to the gear ratio resonating more through the drivetrain. I've heard of 2nd gear not engaging on RF900s - common, but never a B12, especially not third.
Marc/Atlanta