im pritty positive that you have an air leek so the tapping noise is the seelf idiling speed aduster which wount work proply because it needs the rite amout of air which causes the engine to jerk about your engine wile try to stall then the self idiling aduster will try to pick up the reves up put because of the air leek its confused.
So u need to find the air leek and ither tape it up or buy a new pipe.
callum
November 2008