"Yea, they despised the pleasant land, they believed not his word: But murmured in their tents, and harkened not unto the voice of the LORD. Therefore he lifted up his hand against them, to overthrow them in the wilderness." (Psalm 106:24-26)
The dictionary defines murmuring as "a low, indistinct, continuous sound." In other words, spoken so that only a choice few can hear it, said in such a way that one cannot always be sure it was actually said. It is the insidious grumbling of someone too cowardly to speak up, but too conceited to shut up! I say this, knowing full well we are all guilty of it, from time to time. But be that as it may, it still should be recognized for what it is: a form of complaining that will seldom get you into trouble...except with God. (Go back and read verse twenty-six.)
Besides the children of Israel referred to in this passage in Psalms, here are a few others who indulged in this pastime. In Matthew 20:11, we find employees murmuring against their employer because of (perceived) unfair wages. The Pharisees murmured against Jesus and His disciples, because they ate and drank with sinners (Luke 5:30 and 15:2). The same disciples murmured against a woman who dared to "waste" good money, in the form of precious ointment, on Jesus, in Mark 14:5. And the people murmured because Jesus associated with someone of questionable occupation (Luke 19:7). In each case, rumbling resentment was displayed in place of more costly, but biblical, open rebuke (Prov. 27:5). And remember, the ones condemned in our text were murmuring "in their tents." We should not assume that our "homemade" complaints will be overlooked by God. They won't.
Paul cautions against murmuring in two places (1 Cor. 10:10 and Philipp. 2:14), reminding us in one of them that some of those who murmured were "destroyed by the destroyer." God may not deal with you or me as drastically as He did with Israel in the Old Testament, but is that any reason to risk His disfavor?