I've flown on a few commercial flights, and on several of them I recall a phase of the flight during descent when the engine seemed to go idle (or at least, the engine noise subsided substantially) and then all of the sudden, the engine got really loud and I could feel the plane slowing down substantially, as in, I felt the same as if I was in a car that was slowing down suddenly. This seemed to last between 5 and 15 seconds, or possibly slightly longer, and then the engines got more quiet and the noticeable deceleration stopped.
Living under an incoming flight path for a few years I have also noticed many times that a plane will be going overhead by the fact that suddenly the engine will get few loud for about the same amount of time.
This seems to be rather common, and I always assume this was reverse thrust. But today I am reading two separate questions: "Is it possible to use reverse thrust while airborne?" and "Why can't most jets safely use reverse thrust while airborne?", the answers to both of which seem to imply that using reverse thrust while airborne is very rare.
So what have I experienced / heard? I'm pretty sure it's not an airbrake, because far as I can tell a speed brake is a more passive device that doesn't involve the engine, and it would seem to defeat its purpose to spool up the engine while using it. I've never been in a seat where I could actually see the back of an engine to tell if a reverser was deployed, but I think at least in some cases the speed brakes were also deployed; from in front of the wing this made it impossible to see the back of the engine.
Update: Everyone is saying speed brakes... how long are these usually deployed for?