Problem with the timing

Three months or so ago, the otherwise well-running engine developed a
rattling noise. Suddenly, without any apparent cause it began sounding like,
what my mechanic called, “a typewriter”. I drove with the car a little more
but decided to leave it along the road and call the mechanic on Monday. He
said I should take the car to the garage for check-up. Concerned I asked
him if I could still drive the car with that noise in front. He said yes.
After five weeks in the garage and numerous checks: fuel supply, electricity,
hydraulic valves etc. The finally diagnosed the problem to be a defective
timing belt that had skipped a few teeth and thus caused the engine to run
wrongly timed. We purchased a new timing belt, adjusted all necessary
settings and indeed the engine ran beautifully again!
For less than a week I drove the car for maybe 200 km when one day
when I started the car I heard a “clunk” or somewhat heavy knock, after
which the engine started running...... with the same “typewriter” sound
again. I left the car at home and called the mechanic and explained. They
came out and towed the car to the garage where they scratched their
heads and tinkered with the timing, “discovering” eventually that the belt-
tensioner did not work properly. After some deliberations we decided to
replace it. After another week this was accomplished and the engine ran
great again.
I used the car for just two days when the same thing happened: start, clunk
and typewriter! The mechanics came out again, towed the car again and it
is now at the garage waiting for any clever advice that may help solve this
problem.
The mechanic was trained by Volvo and has good experience and Volvo
tools, but has no clue what to do next.
Can you help? Please?
Engr. Huub J. Luyk.
Baguio City, Philippines