If it only happens when backing up, you may have the parking brake engaged.
You forgot to let your wife in
It's really hard to understand what you're asking here."What causes a knocking sound when your car drops into a hole while driving?"If that's your question, I imagine that it would be the sound of your car dropping into a hole.
Gearbox oil needs changing.
Bad CV joint, loose rim, Loose caliper
The combination of fuel still being pumped into the engine and air
Check the drive axle for a broken and leaking boot or bad CV joint.
You need to get your car checked.
Could be brakes or if rear wheel drive could be a "U" joint going bad
Bad ball joint
Because it just does. That's why.
your car being broke get it checked out
Get it fixed.