We call those chemicals "mechanic in a can", and no, additives do not magically fix mechanical problems. If they did, we wouldn't need mechanics.
You need to have someone listen to this to determine the cause, THEN we can talk about the solution. If there are worn bearings in the engine, at a bare minimum they will have to be replaced, but most of the time the crankshaft journals are already damaged too. Those have a mirror finish, and any scratches or gouges will instantly damage new bearings. If this is the cause of the knocking, continuing to run the engine will cause a lot more damage very quickly.
The vibration damper on the front of the engine could be loose too. The pulley that runs the belts is attached to the damper, and the belt tension will keep pulling it up. That makes it knock twice per engine revolution, and it can be fairly loud. That is not serious if it is corrected right away. If it is ignored, if you're lucky, the assembly will fall off and just sit there. Everything belt-driven, like the generator and power steering pump, will stop working. If you're not lucky, it will not fall off. Instead, it will rub back and forth on the snout of the crankshaft and wear it down. At that point you'll need to replace the crankshaft and the damper.
There are even less serious possibilities such as a collapsed engine mount. If one allows one side of the engine to sit too low, an exhaust pipe can bump against the frame or body. That will change between "park", "reverse", and "drive" because the engine rocks in different directions.
Saturday, February 14th, 2015 AT 4:37 PM