Two days ago I turned some brass fittings and everything worked as usual. Yesterday I was honing out a Lee sizing die and started hearing a thump, thump, thump type noise like you'd hear if you had a flat tire. The noise increased with frequency as the Lathe speed would change. What could possibly be causing that? Sounds like it's coming from the spindle area or possibly the gear box. Help me get to the source and fix the problem.