The Pump tends to leak on the timing belt. This causes the belt,
after a surprisingly short amount of time, to become rubbery. The belt
begins to stretch then brakes. Boom valves hit pistons. You are now
in the market for a head plus many other parts.

An easy way to spot a pump that is beginning to leak (front seal I am
referring to) is to make it a habit to look at the left side of the motor
(Jetta & Rabbit/Golf) Front (Dasher/Quantum) at the point where the
Head & Block meet you will see dampness. This is a clue that the
Pump Seal is about to go.