Empty house next door, strange sounds coming from the house, sounds like pipes groaning or creaking. Who do I report it to?

Call the owner of the house.  If you don't know them, call the police and report the strange sounds.   They will come out and check it.  You might go over, knock on the door and look through a window.

Why? ..............................

A priest. Or a wizard, maybe.

start with the police -- possible break in and vandalism

Sounds like you don't own the joint so why do you care?  It is empty.

Sounds like the police need to investigate.

Looters, squatters, meth lab etc all possible

I'd try the police.

Perhaps someone is liberating the house of copper pipes and other cash worthy items to recycle. 

How can you hear pipes groaning from a house next to you?   Are you up against their windows listening in? 

Pipes and heating ducts do make noises. Especially if you are up north and have had both warm and cold weather in the same week. 

Empty? Are the owners on vacation? In between renters? Foreclosure? Up for sale? 

If you know for a fact no one is suppose to be in the home-  perhaps a friend coming in once a day to check,  bring in mail. Or a maintenance/repair/ is in the house with permission.  If you think there is a break in you can call the non emergency number for the police. 

If the house is up for sale/rent call the real estate agent.  Houses in foreclosure  can have a sticker on the door with info. There  could be a contact number.