1991 Acura Legend a 'rattling' noise occurs at 1,500 rpm

1991 Acura Legend L 3.2 V6 mileage: 95,000. For approx. 8,000 miles a 'rattling' noise occurs at 1,500 rpm-sounds like engine is a diesel. Obvious when car is decelerating or held steady at 1500, it runs fine otherwise - gets 24 mpg highway 70-75 mph. The dealer originally said it was carbon buildup and it needed GM Top Engine Cleaner. After they did treatment, car started better but noise is still there - seems to be a little louder. Dealer now says it could be rings but also says 'not to worry about it - it should be good for 200,000 miles'. Any ideas on how to come up with a better diagnosis would be appreciated!