[B. W.]

When you remove the tailshaft housing there should be some shims on the shaft (these shims set end play), then the speedo gear. It is a large spiral cut gear on the tailshaft that is plastic. It mates with the long steel gear you have a pic of.

My first guess would be the speedo cable. I've had more than a few go bad, they usually make a rotational noise rather than a continuous noise, sometimes the speedo needle will jump around also. But, if you had to fix the ratio adapter that could be the problem also.

Disconnect the cable but leave the ratio adapter on & see if the noise goes away. If it doesn't, remove the ratio adapter & check again. that will narrow it down.