Hello,
I had the same problem... every time I pushed the TO/GA button, the warning sound start and last untill "positive rate of climb"... the configuration was "by the manual"... I tried everything to solve, but the only thing that made the warning disappear was setting the speed on MCP to a maximum of 10kts above V2 of the FMC (before I used to set 20kts). I don't know if it is correct... but no more warning for me!