I have an 01 express cube with the 5.7L V8 and 300,000 miles on it. It makes a tapping noise at low idle not when accelerating and driving. Dealership listened with scope and said not related to engine but possibly worn belt or belt tensioner. Are they correct? Also as i was driving on a long trip I noticed the fuel gauge 3 different times went to the empty position for about 20 seconds before going back to giving proper reading. Any ideas there?

Thank you
Marc