For what it is worth:
My adopted sister and I were both raised within (different) practitioner cultures, and raised to be that. Both of us having classic and extreme "shamanic awakening" experiences at young ages. The biggest advantage someone like that has is viewing
everything as a representational system. Including and especially our own meta system. In other words I "believe" both a lot more
AND a hell of lot less than most people.
Mice: statically it probably is. If not mice some other critter. They can get into places you would not suspect. And make more noise by body weight than if you personally were a heavy metal concert.
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Critters ---> structures cooling/settling ---> wind; account for the vast majority of ghost sounds in my experience.
Add in the fact that certain frequencies and combinations of sound not quite in the consciously detectable range of most people can stimulate fear, anxiety, and even cause oscillations in the human eyeball? Now it's a party! (Lets not even get started on the effects poorly shielded EM can have on some people.)
Aside from that: Can't go wrong with smudge.
Rituals work, so don't just play at it. At the same time do NOT wed or overly invest yourself into a specific model of reality or belief system. Do not go down a rabbit hole or, metaphorically speaking, burn down your house because of mice in the walls. If you have any kind of existing believe system, no matter what? Find something that both respects that and that works form a psychiatric model as well.
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Hiring an "expert": Don't. At least not yet.
Best case you will get an experienced and ethical practitioner with some actual skill and ability. They will do a professionally generic dispelling of negative energies in the form of a ritual designed to invest "
you!" in the process. This attacks the problem on multiple levels. Negative energies = actual OMG ghostish what ever! Negative energies = nothing yet, but you looking for whatever is actually ghost bait so stop it. Negative energies = internal psychological dynamics. No ghosts here but you fuck your self up looking for them.
Also an ethical expert will probably suggest doing something about the mice.
Con men, religious kooks, incompetent new agers, pay to play; looong list of NOT the best case. Some of them even quite well meaning.
NOOOOO........ I neeed professional help! I am so freaked out that I'm actually asking a random interweb person called Krazy for advice... on my ghost infestation problems!!!
Eclectic is good. Experienced is good. "Shamanic awakening", the younger the better, is good.
Self appointed Shamans and New age priests are bad.
Given that all you really have are scary sounds in the walls? Unless you know someone you are probably talking pay to play for a very generic hand holding dispelation of bad mojo.
Know someone you might.
If not you definitely know someone that has been helped by a legit practitioner that is a helper type. (reference my comments above on structuring "help" so it works purely from a psychological modality as well before you think "nuh uh".) The problem is getting an actual referral. Also anyone that was all fucked up or having a melt down and then were suddenly better but you never knew quite why? Ask. Many will be a shrink or AA or religion but some will be other.
Finally figured out all the floor boards under the carpet were loose. They would compress when you walked on them and then popup in succession hours later so it sounded like someone walking around. Took a longtime to figure it out too but I got lucky and saw it happen one day. I used to work a lot of nights so my wife was ready to sell by the time I found the problem. lol 
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