"Tommy Knockers" are a type of mining spirit in Cornish folklore, often associated with knocking sounds that would precede a vein of ore. The phrase "Tommy Knockers knocking at my door" suggests a sense of foreboding or warning, as if the spirits are trying to communicate something.
Here are some possibilities:
* Folklore and Mythology: The phrase could be found in books about Cornish folklore, mining traditions, or the supernatural.
* Fiction: It's possible the phrase is used in a book where the supernatural or mining plays a role, but without more context, it's hard to pinpoint a specific work.
If you have more information about the context where you heard this phrase, it might be easier to identify the source.