Speed unrelated suggests a loose A/C belt? That follows RPM.
It revs around 1900 RPM at 60 mph, and just a bit lower than that at 15 mpg if the engine is really cold.
If the belt is shredding that could explain the rattle; the rags are hitting stuff.
Pop the hood, find the belt, check whether it is tight and in good shape or not. Watch it when the engine idles (but keep your hands away!)
If it is the belt that is a worry less and a cheap replacement.
It has been cold here lately and I've been hearing serpentine belt whines on other cars like every week.
2011 Insight Elegance
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Accord horn, Waxoyl, HIDs, Green Oil, UltraGauge, LEDs, full upper&partial lower grille block, aero tweeks, plans for several eco mods
Last edited by RedDevil; 02-02-2017 at 07:21 AM.