Why would a bmx back rim creak?

Answer 1

Having one or more spokes loose can cause creaking. If this is the case take you bmx to your local bike shop. Your wheel may not be true (damaged e.g. fractured/not original shape)

another reason is that your tire may not be pumped up all the way and the inner tube could be chaffing on the outer casing.

Answer 2

for me it means that my crank arm bolt is coming lose, so take out you Alan wrench and tighten them