Hello Brian,
I used to date a girl in Salisbury but that was many, many years ago now.
Your intermittent tapping may be the vacuum pump. It reputedly can do this. If it was valve clearance you would hear it all the time.
Are you sure its not piston slap?
Is it louder when the engine is cold?
Does it disappear if you rev the engine?
This link might help.
http://www.brickboard.com/FAQ/700-90...acuumPumpNoise
cheers,
Steve