This might be something or nothing.
My neighbour had a leaking chimney and he had a few people in to fix it but never did.-Flashing -repointing -spray bricks with water repelant etc.
I looked up at it and noticed he had a chimney cap on top like this one
https://www.burtonroofing.co.uk/roo...dbank-170mm-flue-ventilator-red-rf051087.html
When it rained and especially in a strong wind the water ran off top -down sides of the cap and down into the chimney pot. I said go up there and take it off -put some mastic around rim of pot and put cap back on -Perfect -no problems since.
I had a work mate who had bumping sounds when it was windy -took him years to find it and it was the cap with a uneven base rocking. Some times it is a simple fault but a pig to find.