"Stop grumbling among yourselves,” Jesus answered. “No one can come to me unless the Father who sent me draws them, and I will raise them up at the last day."
This is not something to be grumbling about; the facts cannot be changed by argument. John continues along his strong Calvinist line. Here is total depravity. You can't come to Christ unless the Father draws you because every part of your being is turned against him. Yet in 5:40 John has Jesus blaming them for refusing to come to Jesus and have life - human responsibility.
Divine sovereignty and human responsibility are firm friends, not enemies.
Grumbling is a horrible sin. It is a symptom of not accepting; of wanting to be right in ourselves. We should accept it when we are in the wrong and make it up with those who are in the right.