If you're hearing knocking, answer the door?
This may be the least descriptive post I've ever seen....
We need way more details.
What year is your car?
What trim is your car?
Auto or Manual?
Check Engine Light (CEL) on?
Any mods? Stock motor?
Where's the noise coming from?
When does it happen? When turning? When straight? When braking? When on the gas?
Intermittent or constant?
The more details the better.