What is causing the rattling/knocking engine sound in my 1997 Mercury Grand Marquis?

[Daniel]

Hey, I have a 1997 Mercury Grand Marquis with the 4.6 V8 engine. For a while now, the engine has been making a fairly loud rattling/knocking noise when the engine is warm and under a load, especially when the transmission is in third gear and vehicle speed is around 30MPH. Also, the sound is considerably louder with the Air Conditioning on. I tried using 10W40 oil instead of 10W30, but this didn't help any. The owner's manual recommends 5W30, but Haynes Repair Manual recommends 10W30. I've heard that this sound can be caused by detonation/preignition. I'm going to try some fuel injector cleaners, as well as switch to a higher octane fuel. In the meantime, anybody know exactly what's going on? What's causing that noise? Thanks!