This typically indicates a loose belt condition either on the generator or counter shaft belt.

You should check the belt tension from the engine drive to the counter shaft belt or the counter shaft belt to the generator.

It may indicate low RPM on the engine. Check that the engine RPM setting is correct (1800 to 1840).

It may indicate a low voltage situation. Check your three-phase voltage at the main disconnect to see if you have 480 volts on all 3 phases.