The Electra Hellbilly has a three piece crank/bottome bracket. There are many things that can cause a knocking noise on your bike. I will give you a few places that you can check but you may need to take it to your local bike shop.
1st. Check the pedals. Take a 15 mm open end wrench or pedal wrench and makes sure the pedal are tight. Also check the pedals for any play in the bearings or tight spots. If they seem to have any issues, try a new set of pedals. Test ride.
2nd. Make sure the crankarms are tight. Use a 14mm socket wrench. You may need to remove a cover over the nut with a small flat head screwdrive. If you have a torque wrench, they should be between 25 and 35 ft lbs. Test ride.
3rd. Check the chain tightness. It should have just a small amount of play in the chain. Also check the chain for tight links or bad links. Do this by either flipping the bike, putting it in a stand or having someone hold the rear of the bike up while pedaling and watching for the chain to not flow over the cog in the like the rest of the links. If you find a tight link you will have the work it loose by hand, going back and forth with the tight link your hand and lubing it with something like tri-flow. Test Ride.
4th. The last two solutions are better off adjusted but your local bike shop. You need to check the bottom bracket and rear hub. We can start with the hub. While standing over the rear wheel, move it side to side. If you feel any play then your hub is out of adjustment. Adjusting the hub requires cone wrenches so unless you have the tools it will be less expensive to have it done. For the bottom bracket, grab one of the crankarms and move it side to side, if there is any play here then it requires adjustment. Once again it requires special bicycle specific tools so it would be less expensive just to have the repair done.
Hope this helps. Let me know if you need any other help. You can contact me at firstname.lastname@example.org
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