An exhaust leak can sound like a lifter (or valve train) noise.
A loose spark plug (compression leak) can sound like an exhaust leak.
Either of the three (exhaust, spark plug, or valve train) can usually be fixed without too much pain.
However, the "knock" you mentioned is a potential killer. If you have a lower end (rod) knock, all of the screwdrivers and stethoscopes on this planet aren't going to do you much good.
I think you should follow the first post's advice, start with checking the oil level.
Let's face it, if you have to rebuild, you can always punch out a few more cubes.