Ronnie, I'm going to expand on my original post under the pretense that this isn't all complete bullshit. I believe you are telling the truth, however.
I hope you realise by now that this isn't your average haunting, What you are dealing with is most certainly demonic in nature, As in a spirit who has never walked the earth in human form. It may sound outrageous and hard to believe but your occurences and experiences at this house are typical of these hauntings, All the warning signs are there just from reading your OP. Despite all the jokes going on this thread, I urge you to NOT be taking this lightly at all. This type of stuff can ruin your life if you allow the situation to escalate any further. You could lose your sanity and even your life.
I know how frustrating it can be to be told that what is happening to you isn't real or it's just all in your head, The fact of the matter is most people simply cannot comprehend your situation until it happens to them, Which I wouldn't wish upon anyone.
I believe this is a demonic haunting because;
A. Voices being mimiced - If you hear voices of a different tone of which sounds like a stranger, then it could be considered a regular haunting. However, you claim the voices imitate you and your girlfriend. These are usually the first warning sign of a demonic haunting, It is mocking you, If you had kids it would certainly imitate their voices too, Which is when it really strikes close to the heart. You also said you heard the voice of a girl, maybe even a child? this is usually typical behaviour of demonic activity too, This presence at your house is trying to fool you in to believing it is innocent and harmless. Don't fall for it.
B. Strange writing - This is something I would like to see photos of so it can be deciphered. The triangle you speak of is what concerns me the most, It could be part of a reversed pentagram, which is an evil symbol. The writing could be latin, which is also demonic in nature. Either way, we won't know until we see evidence.
Like I said in my previous post, Simply moving house may not stop this from happening. Demons are not drawn to property, they're drawn to people. What I suggest is to contact a priest and peform a blessing on the house, this may reduce activity for a while but even then it may not fully come to a stop. Also, DO NOT under any circumstances attempt to communicate with it. I know alot of posters in this thread suggested just that but trust me, it will only make things worst for you. It WANTS you to communicate with it.
Things you should look out for is anything coming in three's. Such as hearing a knock three times in a row, or having three scratch marks on your body of unexplained origin. If you start to notice this, then it all but confirms suspicions if you had any prior doubt (if thats even possible). The reason for this is that anything that comes in three's is considered to be the mocking of the trinity, another sign of demonic activity.
Anyway I hope this info has helped. I'm sure this post will be laughed at by the narrow minded posters but i honestly dont care. You should figure all this out and do what's best for you. This is your life you'r in a nasty situation.