Detonation???

Hi again folks! 1997 GT stock set up aside from 3.73 gears and a short throw shift. I noticed tonight on my way to work that in 4th gear, around 4000 RPM, if I floor it to WOT I hear a mild "rattle" coming from what sounds like the undercarriage. Performance has not changed, fuel consumption has not changed. No ticking or knocking while idling. It does it regardless of whether the AC is on or not. It sounds kind of like when you are in too high of a gear at too low of a speed. Is this "detonation"? If so, why would it only do it in this gear? I don't want to screw up my engine. Please advise. Thanks in advance!

Side note:
I tried to get audio recording but it is too minimal for my crappy Metro PCS phone to pick up.

She is stock - no tune. I recently used low grade fuel whereas I normally run 91 octane. I assumed it would call for 91 since my Mom's old Mark VIII required it, and the engines are nearly identical (granted to the Mark VIII is the Cobra motor vs the run of the mill 4.6). Would you suggest running the tank dry and refueling with a higher grade? Thanks bud. It was really weighing on my mind since I got to work. In between patients I have been researching and reading allllll of these horror stories about detonation and combustion issues.