Recently bought a 2008 ford f150 off marketplace, having this strange issue where every time I reverse, and maybe 30% of the time when I idle the truck will start knocking and shaking a bit and if I let it go to long it will stall out and the battery light will come on. There are no lights on the dash except the tire pressure and airbags so probably not related. Wondering what could possibly be causing this issue? If i shut the truck off and turn it right back on when it starts knocking it runs fine, and it never does it if I reverse immediately when starting the truck. Clean inspection ticket and nothing showing up on my code reader.

 

Guessing it's a 5.4 liter? Try unplugging the vct solenoids next time it's acting up.
Fairly common for the 5.4 to have issues like this especially if the miles are high and or oil pressure is lacking like at low rpm situations like reversing and idling.