My problem is a metallic rattle that I hear only when accelerating, and even then when pushing it a little 'extra'. Noise not present except when in D and leaving a stop. Otherwise car is fine, ie no loss of power or shift problems.

The closest thing I have ever heard to this is in a Std car I had (VW) If mistakenly you started from a stop in 3rd, the tranny or clutch struggled and rattled until speed was sufficient for that gear, if that helps...

Any thoughts?

Heat shield on the cat may be loose if the welds rusted through.

Steve

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'91 MB 190E 2.3
'08 RAV4 Ltd 3.5
'83 Lazy Daze m'home 5.7

I agree, likely loose heat shield. Raise car up safely on Jack stands rated at least 5 tons each. Slide under car and inspect the exhaust for loose heat shields . Most likely ove the catalyst.

Are you sure it is not fuel quality related detonation (spark knock). It will sound a lot like a loose shield.

it could be possible like some of trhe guys say about the heat shield.but i bet u it is the sepintine belt tensioner shock.i bet when u open the hood and see the black shock u could move it back and forth by hand.

I had a very slight vibration only during hard accel. Motor mounts got rid of it. Maybe your motor mounts are at the point where they transmit just enough vibes to make things rattle.