Timing chains like their rubber cam belt counterparts should be changed at certain mileages, the M102 should be done at about 160 000 kms if not sooner. These are single row chains and the links wear (quicker if oil changes are not done regularly) and thus the chain appears to stretch. If the chain jumps a few teeth say goodbye valves, and if it should break then goodbye engine.

Not too difficult a job to replace if you have a workshop manual of sorts to follow, also some excellent articles on this forum under DIY LINKS.

Do it sooner than later!