when i started up my car and was getting ready to drive away i heard a knocking on 2000 Ford Focus

sound like a rock that hits the bottom of your car but there is was not rock. What could of made that sound?

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3 answers
Broken engine mount??? .....just one more guess.......should get it looked at. IF in doubt -tow it!
Did it go away or still knocking? Oil level and pressure ok? Several poss. causes get mechanic to take a look!
ball joints or sway bar end links