Wel you will almost always find some metal in the filter and oil but usally a very small amount. Something is out of whack. Changing the cam will only solve the issue if the current cam is bad. Same for the heads.
At this point sounds like you need another set of eyes on the issue. Got any freinds that are good techs or work in a shop. A fresh look at the problem will usally solve the issue.
I had a crazy electrical problem. I could not solve for the life of me. Took it to a shop and they messed it it for an hour then pulled the engine and went of the wires until the found the issue. Bad connector. I had never pulled apart because it looked good and not related to the issue. Once thye got everything apart the problem was appeaernt and they had the car fixed in a few hours. I spent a week testing crap. Thats when i decided i hate cars with comuters. Especially if its a ford with anti lock breaks that go out randomly. And when it does you can not push the peddle hard enpugh to stop the car. What an awesome design. When it fails you have no breaks. Ford os also the only car i ever owned where the floor mat could get stuck under the gas pedal and keep it floored after you let off.
So another pair of eyes at this point would help alot. That new view on the problem my result in a quick fix. It is not right that it is knocking all of a sudden for no reason out of the blue. There must be another problem. The way you described the issue. I have not seen any type of detenation. That bad before.