WD-40 stands for Water Displacement Formula
#40 . it's NOT a good lube.
Being that the noise sounds like the chain on the rerar cogs I'd guess that somehow your rear derailler got out of allignment and now your indexing (it is an indexed system?) is not spot on.
Is this happenning with both chainrings on the front? Is your frunt derailler touching the chain
Do you have 7 cogs in back and 2 chainrings up front?
Cheers