Q: When pressing the gas pedal/accelerator, from 0-40 mph, there's rattling noise on 2001 Nissan Xterra

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When first accelerating, there is a rattling noise, similar to vavles ratting, sounds like underneath the front
(2) Answers
or you can check for loose /rusted exhaust heat shields. Will it make the same rattle if your going up a steep hill at a steady 30 MPH and then accelerate in the same manner (moderate or hard on the throttle you don't say) same symptoms cold or warm engine ?FULL tank of premium (I like sunoco -ultra when available) see if that makes a difference then we can go right back to the detonation or 'spark knock' and determine the correct coarse of action. You don't have a gravel driveway , do you?