Mountain Bike Chain Is Loose. Bikes without a derailleur are single-speed bikes or fixies. A slackened chain will decrease your ability to transfer energy to the pedals of your mountain bike.
A majority of BMX bike chains are much narrower than road or mountain bikes; BMX chains measure at ⅛" width. To fix a slipped bike chain, first, turn your bike upside down, as this will make it a lot easier to reattach the chain. You'll more than likely find your chain is loose and you'll need to tension it.
If too tight, you will find it difficult or impossible to shift down to the small chainwheel.
Related: How to Lubricate Your Bike.
I noticed that if I changed the twist of the blade section (by turning the spoke slightly with an adjustable wrench set to just slip over the spoke), I could eliminate the noise. It's easy to diagnose a loose cassette – hold the rear wheel in one hand, and the cassette in the other, and slide it along the axle. Because the cogs are steel and if they're really loose they'll be jumping around enough to sound like rocks in a rolling tin can.