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New knocking noise after repair - a problem?

I recently had the distributor and coil replaced in my 94 Celica. Since then, there is a slight knocking noise that lasts for a few minutes when I start the engine cold. I took it back to the shop that did the repairs and left it there overnight for them to listen to the noise. The assured me that this is normal wear for an old engine with 142000 miles on it, and told me to switch to high mileage oil and add engine treatment when I change the oil to prolong the engine life.

Is this something to worry about? Is it likely a conincidence that the knocking began after the repair, or are they trying to pull one over on me?

The old distributor near the end of its life might have been delivering a spark that was retarded. The new parts restoring a strong spark at the proper timing would put more stress on the old engine and cause worn bearings or loose pistons to rattle.