Why is my xbox making a knocking noise?

There is a knock. Also it fails to launch games half the time. I got an expanded memory drive. It helps a little. Its still a loud noise.

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what are you doing when the knocking happens?

do you start it up and it knocks?

insert a disk and it knocks?

start up a game and the fan spins up and it knocks?

any amplifying info would help troubleshoot this a bit more.

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