I can here a intermittent ratter ling noise when I acerlerate but not when I am going at a constant speed.I believe the engine is using slightly more fuel.The car has done 98,000 miles and it is a automatic Nissan w reg

The noise may be engine knock. Higher octane gas will likely eliminate the noise. If that solves the problem, a cheaper long-term solution may be to install colder running NGK spark plugs.