Mouse in cavity walls

[Caroline]

Hello
I seem to have a mouse or some other kind of animal inside my walls and I have no idea how to get rid of it. I have searched this and other sites to try and find how I can solve this problem and the answers that are always given are:
A - Use poison - but then you are left with a smelly, rotting corpse in your walls.
B - Use one of those ultra sonic devices that is supposed to scare them off - but all the reviews of those devices say they don't work.
Or C - Put a trap next to the holes - but I can't find any holes.
The noise is only downstairs so I don't think it can have come in through my roof and it doesn't appear to have come inside the house but must be coming straight into the cavity walls from outside so it'll do no good just dotting traps around my house in the hope a mouse does venture towards it.
Does anyone have any suggestions?
Thank you

[NazNomad]

It has to have got in somewhere ... around a pipe, gap under a window, etc etc.

Your average mouse can fit through a penny sized hole.

My neighbour has an ultrasonic plug-in thingy, swears by it ... try one.

[gaspanic]

2 options

1. name it stuart and feed him bad cheese and listen to his pitter patter and saw awww
or
2.get a sledgehammer and slam the sh1t out of that wall crumbling it down leaving his torso and spin ripped in two silencing his noise and helping you carry on life mouse free

[Arksided]

Ive tried those plug-in things they didn't seem to work for me. With the house next door to me been empty (they had pets usual food for varmints) and they ventured into my home. (looking for food) I have since caught five of them using the 'link' method.

http://journal.chrisg.../how_to_catch_a_.html

[theshedman]

If it's in the cavity and is not coming into the house it must be going out to get food as it won't find any in the cavity. Find the hole, hopefully when it's gone out for food and block it up. You need to see to it soon or you could end up with a lot of mice. As NazNomad says they only need very small hole to get into the cavity. Just a case of finding it.

[ladybirder]

Err I don't think you'll have just one. Sorry.