Okay have a 84 ford f150 with a 351 . 4 wheel drive. Sometimes I go to start it up and it starts fine, and fast. Then sometimes I get in and try to start it up and there is this God awful screeching noise and nothing. Along the line of trying to figure this out, I ended up having a 2 foot long 4x4 wedged up in between my starter and the oil pan. . And for some reason whatever space its causing or holding whatever in place, it will start up again just fine. Every drive or two, this happens and I have to get out and get under the truck with a hammer and smack this 4x4 upward a few times and then it'll start again.. this sounds crazy to me...but can anyone tell me what this is about or how to fix it

 

I'm just guessing here, but it sounds like you maybe have a bad spot on the flywheel or ring gear. The 2x4 between the starter and the oil pan is pushing the starter a little closer/tighter to the flywheel, which helps it start when it's on the bad spot.