2.5i premium 5 door, 5 speed. Intermittent light engine knock.

What is your question?

I have a 2010 Subaru Impreza 2.5i premium 5 door, 5 speed with 8k mls on it. The car runs well and does not have any check engine lights on but there is a slight intermittent knock when switching gears and staring off in first. I was told by my Subaru service manager that the car only needs 87 octane and that I can experience occasional knock and that it is normal. I have been driving manuals for 25 years (both foreign and domestic) and never had this happen. Is this an issue I should push with the dealer and will it eventually destroy my engine? I was hoping to get at least 200k mls out of this car.