Noise coming from valve cover

The alternator stopped working on my 1998 Ford Expedition. This happened at night and I had to drive approximately 15 or 20 miles on battery power.
I have replaced the alternator but now there is a slapping sound coming from the front of the right valve cover. I have placed my hand on the valve cover and I can feel a vibration as if the timming chain is hitting against the valve cover. If the timming chain is hitting the valve cover, what could be the cause? Sorry, I forgot to mention that the engine is a V8.
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