Your problem is worn synchro rings from gears 1->2. The synchros are designed to match the tranny shaft speeds (input/output) before the actual gears engage, this keeps things quiet. To repair this the transmission must be pulled apart and syncro ring(s) replaced. Not a small job, and might as well use a new clutch during re-installation.
Mike McMahon, Baltimore MD
1970 280SL 4-speed manual
olive green ext, bamboo int