Inspect the belt closely. If the following problems are evident, replace the belt with a new one.
Hardened back surface of rubber
Back surface is glossn, non-elastic and so hard that when the nail of your finger is pressed into it, no mark is produced.
Cracked back surface of rubber.
Side of belt is badly worn.
A belt in good condition should have clear-cut sides as if it were cut with a sharp knife.
Teeth are badly worn out.
Missing tooth
If backlash or an irregular noise is observed when rotating the pulley, replace the timing belt tensioner and idler pulley.