Rougher than normal engine operation:
After thousands of miles of operation the teeth on the timing belt can worn down or break. The belt itself could also stretch, this can cause the belt to ‘jump’ position on the crankshaft. If this occurs you will see a noticeable difference in the performance of the engine.
Engine will not start or stops abruptly:
On occasion a timing belt can jump or break even while the engine is running.
Knocking or clanking noise:
Should a timing belt jump, the pistons and valves could collide, this is the knocking noise you will hear, if you carry on driving it could cause considerable damage to other areas of the engine.
Not replacing as per manufacturers guidelines:
The timing belt should be replaced at the service interval as specified by your vehicle manufacturer (regardless of any visible problem or not). This is typically in the range of 70K – 100K miles.