Engine Knocking 2002 Nissan Maxima

I drive a 2002 Nissan Maxima SE model with 30K. About a week ago I noticed a light knocking when I accelarate between 2K to 3K rpm and at most times same noise comes up during sudden acceleration between 0-20 then 20-40 mph. I went to the Nissan dealer in Spokane and I was told it will cost me roughly $80 for diagnostic test. Does that sound about right? What's really causing the engine to knock at a very low mileage. What is your recommended fix to this problem? The owner's manual is recommending fuel w/ 87 or higher octane level. What about "Fuel Additives?" Will fuel additives fix the engine knocking? Please help.

Felix H.