Considering the proximity of the air box to the timing chain, I can understand the mechanic's dilemma. No, I would not expect a mechanic to open the air box when I bring a car in for unusual noises in the engine compartment, why would he? But that would be the first thing he would remove if told to replace the timing chain. An honest mechanic would have cleaned the air box, called the owner to pick up the car, and charged just 1 hour of labor. An unscrupulous mechanic would have kept the car for 3 days, cleaned off the side of the engine, and then presented the owner with a $3,000 bill.

Sounds like you have found the source of the noise! Good for you! Now, clean the rest of the engine bay and inspect all of the exposed wiring looking for chew marks. And tell the previous owner what you found. He may have the same problem with other cars/trucks.
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Keeping it running is better than buying new