Based on personal experience and investigations
It was a pounding. My voice recorder would catch it at some investigations, but it was never something I heard with my own ears. The pounding sounded so far away, but close because of the strength behind it. This was no normal knocking. I always thought of Jack and the Beanstalk, the giant pounding at the door to be let in. Since it was no big wow, I ignored it, giving my attention to the words of EVP's.
While ghost hunting, if I see something, I will mark it on my voice recorder. That’s how I learned ghosts can hear it too. I saw a ghost of a older woman watching me at an investigation at Waverly Hills Hospital. As I stared open mouth, I saw her head whip to the left, searching and when she looked back at me, I saw fear in her face. She vanished. I was confused, it was like she had ran. A ghost running? I wanted to know what scares a ghost because, well, you know, they were dead.
A few days later, while listening for EVP's from the investigation, my voice recorded seeing the ghost of the woman and a second later, I heard the pounding. Then my voice describing her reaction. That is how I learned she had heard the pounding and it's what scared her. What in the world could be pounding and why? I'm finally paying attention to it and found an explanation. Basically, the walls are being made weak by energy, some want out. Not good.