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Hello, my name is Bobby. Thank you for the question. Does the noise occur with the a/c turned off?
The drag from the a/c compressor should not be enough to cause the valve to chatter. If that is the case you should be able to get the same chatter if you hold the brake pedal down and give it some gas. If it will not make noise at any other time then it sounds like the a/c compressor. I have heard this compressor make noise that sounds just like a valve train issue.
Let me know as this will tell us a little more as to what is going on. I have seen more compressors cause this than anything else.