When driving i hear a knocking sound on my mercury villager 2000

Could be a few things but if it totally goes away when you hit the brakes, I'd start by inspecting the calipers. Make sure the bolts are tight. This is just a start though, you need to perform a complete inspection of the steering and suspension. This could also be an axle. If you don't work on cars, then use this link to find a good shop near you. https://repairpal.com/repair-shops Alex

Agree - indicative of loose brake component(s) ..