The spiritual meaning of “grumbling” or “murmuring” in the Bible is, according to Swedenborg, pretty straightforward; it represents complaining — but specifically complaining brought about by temptation, or spiritual challenge.
Passages from Swedenborg:
Arcana Coelestia (Elliott) n. 8351
8351. ‘And the people grumbled against Moses’ means grief caused by the bitterness of the temptation. This is clear from the meaning of ‘grumbling’ as complaint, the kind made in temptations, thus grief caused by the bitterness of the temptation. The temptations which those belonging to the Lord’s spiritual Church underwent after they had been delivered from molestations, in addition to the temptations which members of that Church are destined to undergo, are described by the grumblings of the children of Israel in the wilderness. And since spiritual temptations are as a general rule protracted till a person is in despair, 1787, 2694, 5279, 5280, 7147, 7166, 8165, ‘grumbling’ means complaint because of grief felt in temptations, as in Exod. 16:2, 3; 17:3; Num. 14:27, 29, 36; 16:11. The words ‘against Moses’ are used because it was a grumbling against what was Divine; for ‘Moses’ represents Divine Truth, 6723, 6752, 6771, 6827, 7010, 7014, 7089, 7382.