Note that I discovered this the day after making the horrible mistake of listening to my family about riding in wet, muddy conditions when my bike is not properly equipped for such terrain.
My problem is the gear setting is off from where the chain is, my bike is 21 speed, but here's the example:
Setting 6, chain is in sprocket 7, a light rattly sound can be heard when moving the pedals.
Setting 7, which is where it should be regarding the chain's location, eliminates the rattling sound. Switching back to set 6 does not move the chain to sproc. 6, it remains in 7 and the rattling returns.
A similar problem with the lower gears. Setting 1, chain is in sprocket 2 and rattling, etc.
I'm not fully sure on what the cause may be, but I have slight suspicions of mud being clogged somewhere.
I tried manually moving the chain and trying manipulate the gears and hoping it would 'sync', but no go. Basically, the chain is one gear lower than the setting. It wasn't like this during my brief ride yesterday.