There is a strange crackling, raining type of sound coming from behind one of my bathroom walls. I am worried is it electrical or what is it? Is it dangerous? It just started one month ago.

Is the noise constant? Is there anything in the immediate vicinity of the noise? For example a wall switch, receptacle, or sink?

Also, it is possible that a "critter" has gotten down inside the wall cavity.

They make devices that allow you to see through the wall. I think there is one that connects to a cell phone on TV.

Or failing that you might be able to rent a tool from Lowe's or Home Depot that has you make a tiny hole in the wall and insert a camera into the cavity to see what's going on.

Try turning off water, then Electric, then circuits 1 by 1 to further locate if electrical.

The device you are looking for is a USB endoscope, and there are free and paid apps that you use with them. The scope costs less than $20 and is a very handy device to have around. THey are less than 1/4" in diameter and many are 6 - 10 feet long.

Is it the wall where the plumbing is located?