Why does my dvd player make a noise when I put it on top of my old tv?

When I lay the dvd player facing me on the top of my tv it makes a vibration noise but when I turn it vertical it stops. What is this? Its not gravity cause I had a stack of dvds on top to test it when it was on top of my tv. When I removed them the sound was still there. Anyone know what I am talking about?
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