How far out from landing does the gear come down?
It depends on the aircraft type, partly, as to what the SOP is in terms of gear coming down. It can be used as a tool to slow down an aircraft in addition to the flaps and spoilers/air brakes.
On an Airbus A319/20/21 it is very easy to tell when the gear is deployed. You can feel the aircraft slow significantly and there's a sensation of the aircraft's tail wanting to rise.
A common sensation of rumbling about 6 minutes away from landing would have been the deployment of spoilers which can cause some cabin noise (the airflow over the wing becomes turbulent and noisy).