Pleased that it has put your mind at rest, there is some vibration through the chassis, it generally not that noticeable unless you look for it.
A couple weeks ago I had an annoying rattle, which is not usual as the car is very quiet, I couldn’t understand where it was coming from, tried everything to find where it as coming from, as you do.
Turned out it was the spring recoil tape measure that I had put on the front passengers seat and the vibration coming through the seat caused the spring inside tape measure to rattle, when I caught on and moved it, the usual peace and quite was restored.
Some vibration is inevitably I suppose.