When anything mechanical starts making unfamiliar noises, it's best to pull over and take a look. Oil on the ground(or splashed back on the tire) is a NO GO indicator....phone a friend with a trailer, call a wrecker, or start pushing.
I bet the mechanic pooched the counter shaft seal install as I have read it can be a delicate thing to get in without damaging it. There still should be plenty of oil left in the gearcase, so fix the seal, fill 'er back up with oil and see if the noise is still there. You may or may not have damaged the gears by riding on it with no oil keeping them coated.
"You do your own thing in your own time"