1997 Ford Festiva Tapping noise in engine.

Engine Mechanical problem
1997 Ford Festiva 4 cyl Front Wheel Drive Automatic

Hi, hope you can help me here.
My Festiva has an occasional tapping noise in the engine.
It sounds for all the world like a loose tappet adjustment, but that cannot be so as the tappets are hydraulic.
The other strange thing is that if I am cruising at say 40Km /Hr when the tapping is loudest, if I quickly flick the ignition off and on again, the noise is gone and may not come back for quite some time.
If I drive at over about 60Km/Hr I never hear the tapping.
Could it be something electric, like perhaps a faulty fuel pump or something?