They had a failure with an engine, and the flight crew shut it down. But the passenger I knew was shaken up by the experience. Another passenger was unaware of the problem. I was meeting the passenger who heard the sound at the airport, and the other passenger gave some more details.
It would be disconcerting but if you have to shut one down, altitude is the place to do it. You don't need all that power after you get to cruise. The possible missed approach is the real concern with one engine (on a two engine airplane).
A non-catastrophic engine failure is going to sound pretty much like normal shutdown on the ramp. A whining sound that becomes less and less as the engine winds down. To the trained ear it would be fairly obvious, especially if both of the engines were winding down. If airborne, the sound of silence would be deafening, except the screaming in the passenger cabin, might be louder.