murmur
a mumbled or private expression of discontent;  to speak in a low tone or indistinctly.

complain
an expression of discontent, regret, pain, censure, resentment, or grief; lament; fault finding:

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Conclusion:

When murmuring occurs in the Bible Scriptures, those murmuring are rejecting God/Jesus and trying to replace him or remove him or his influenence.  They wanted to choose a leader to lead them back to captivity in Egypt.

In Heaven, the complaining or the Angels led to the action of trying to overthrow God and replace Him with Saton whom they thought would be a better leader.

When the Jewish Religious Leaders murmured, they wanted to terminate Jesus’ influence on the people and replace Him with their own influence for reasons of power and greed.

When murmur ocurs in the Scriptures, the murmur is against God’s Will and/or against God/Jesus.

We all complain about things and may even find things in Heaven in Eternity that we wish were different;  however, such a compaint should NOT be against God’s Will or to replace Him with something else.

You can bring a problem to God and discuss it with Him respectively presenting your viewpoint, but His decision is final and best for you as well as compatible with God’s Will.  God knows what is best and considers results, and long term effects which we cannot — now or in eternity.

References:

Bible Search Source:
https://www.biblegateway.com/quicksearch/?quicksearch=murmur&qs_version=KJV

Definition of Words:
http://www.dictionary.com