STP is the wrong thing to use for a clattering lifter. Try a can of Sea Foam, Berryman B-12, or even Marvel Mystery Oil.
You did not state if one or more lifters are rattling or if this rattle is constant or not. The rattling could be caused by faulty lifters (they must all be replaced) or by a number of gummed up or stuck lifters (a cleaner may fix that).
Your mechanic is incorrect that rattling lifters cannot cause further damage. If left alone loose lifters will also destroy cam lobes. That rattling is actually sledge hammer blows being applied to the cam lobes and when they beat through the hard coating of the cam lobes the camshaft will not last long.