Wow, this brings back memories.
I have actually heard this before. Someone called in to art bell like ten years ago and asked this. Several people called in later and confirmed that this was a thing.
I don't remember the details. Some people called in 'knocking' and I don't remember what caused it. It wasn't the actual witch or there craft. It was something else.
 it is also nothing new. There are accounts of the knocking outside phenomenon as far back as Carl Jung. https://books.google.com/books?id=usrGSaO7QosC&pg=PA64&lpg=PA64&dq=paranormal+hammering+knocking+outside&source=bl&ots=zC6_giJRF6&sig=-rmNU3aGhZk9SEBz6JvROYeqYrM&hl=en&sa=X&ei=A20HVeuVNNGyogSo5YC4BA&ved=0CDw...
He's somewhat of a celebrity heh. There's this book about him.
If other people are able to see this thing, then it's either 1. a powerful thought-form that Sam has unconsciously created, or 2. an entity that has latched itself onto him (and probably not a very nice one - could be a parasite - the 'rubbery feeling' part of this sounds very familiar to me). You could also cross post to r/occult for a different perspective on this. We deal with entities like this all the time. If Sam wants to get rid of George, there are methods and basic information available that he could try, but they're not easy and he'd likely require assistance for anything more advanced than what you'll find in a book.
No problem! It's rare I get to recommend books and such like this.
This is a 3 in 1 collection of my favorite horror manga (and possibly one of my favorite horror stories in general), Uzumaki. It's a story about a curse on a town, causing spirals (the spiral shape is called an uzumaki) to appear everywhere, and slowly everything falls apart. Really a great read, it stuck with me after finishing it.
This may be my favorite j-horror anthology in my collection. It's cheap, but it's also better than many others out there. Each story is between 5 and 20 minutes long, with a total of 60 stories. While some of the stories are better than others, there really aren't any that are bad, like in some anthology collections. Easily worth the price.
I hope you enjoy them!