Replacing spark plug wires

Could be a variety of things, major or minor. Exhaust leak, lifter, rocker...

Use a screwdriver, pipe or hose. Put it up to your ear and probe around to pinpoint where it's coming from.

You can use a store reciept or thin paper to find an exhaust leak. Move it around close to the manifolds with it running. You will see the paper pulse if it''s leaking.