Valve rattle?

I have an automatic Ford Escort ZX2 '98, 91.000 miles. I am having this rattle-like noise as I drive, usually it occurs on acceleration or when driving uphill. What can cause this noise/rat-a-tat-tat-rattle/pinging? How much is it to fix at the car repair? Thanks!

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Clean the MAF, change the fuel filter and plugs. It can be caused by lean mixture, EGR not functioning correctly, timing incorrect, engine overheating.

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Or maybe he just has a loose converter shield that rattles as he accelerates. Cost=0

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