Finally - My boiler system automatically runs it's fan and pump once per day, to ensure that their motors do not seize - no matter whether they have been used, or not. Might that be what you can hear?
Sounds as if it is haunted then as that is all that’s left. Have a look in the loft to see if anything has come loose, tiles or some such, check the guttering from outside. If you cant see anything, sell up and pass the problem to someone else.
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