Okay this might be in left field...but are you sure it's coming from the engine side and not something rattling in the interior/under the dash/window components/etc?
Few years ago my car had a nasty rattle, consistent with RPM but only from idle to about 2000.
I was going crazy trying to pinpoint it while it was happening....finally one day I went into the trunk with a flashlight and mirror, checking all the nooks & crannies. Wedged down on the sides of the trunk floor, down between the inner and outer quarter panel skins, was all this...
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