metallic clank when shift from 2-1

[patagrande]

Yesterday I put in a new motor mount in my '83 300SD, and also checked the valve adjustments. Today when I slow down and the tranny shifts fom second to first, there is a loud "clang." What gives? Could I have misallaigned something when I jacked up the motor to get the new mount in there? The car has 193,000k and was shifting perfectly until today.