When driving at slower speeds (haven't been over 35 yet as just recently insured), whenever i release the throttle the car starts to make a knocking sound coming from the front, but my dad says it sounds like it's from the rear. When the car was getting test driven, there was no knocking sounds whatsoever. What should me and my dad be checking, and how?

Check gearbox / engine mounts or exhaust mounts (front and rear)

I'll add anti-roll-bar link arms to that list (most common and possibly cheapest fix).

What would be the best way in checking the gearbox/engine mount, and do you have an image of what it looks like so i know when i go to get a new one