Don't get to comfy with the noise going away.My 99 started this sympton at 4k miles.I use 5W-30 oil and FL-820S filters.The problem is not as noticeable in warm summer months.I have 14k miles on the car now and it still does it every now and then.I talked to a Duratec engineer at Ford.He has heard the engine noise in my car.He is puzzled by it.The dealer has done nothing.It never acts up there.I have the 6 year 100k mile insurance policy on the car.I will get it fixed sooner or later....
