I used to hear a strangled muffled male voice when I lived with my parents. No matter how much I searched I couldn't find a source and when I tried contacting a spirit I never got a response.
The first time I heard the voice (age 15) I had just walked out of my office and into my bedroom across the hall. My parents had both gone to the store and I didn't have a tv/computer/radio on. I searched to see if someone was in the house and it was just me. I'd hear it every so often when I'd walk out of my office but could never make out what it was saying. The voice always sounded the same and sounded kind of distant. It eventually stopped after a few months.
A few years later (age 17) I heard the voice again. This time it was while I was in the shower. I about fell down every time I heard it because it seemed like the voice was in the shower with me. Again I searched the house and found nothing. I never felt anything touching me, just the muffled voice. It got to the point I'd hear the voice almost every time I took a shower. Finally after about 7-8 weeks it stopped again.
Now I'm hearing it again (age 24). It seems to be at random locations and occasionally I'll hear the voice when other people are around. It sounds like its coming from right next to me but I still can't make out what it is saying.
I don't feel threatened by the voice, I actually feel kind of bad that I can't understand it.