Before you get too carried away with strange engine knocks check your engine mounts, if they are the round ones from the GT/GTB, & they have split it will sound exactly the same as a big end knock, get under there with a jack & have a looksee before you make the same mistake as 2 others I know it's happened to.

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Beaten up old 98 Gen 2 Liberty, 13.0, stock V3 STI running gear.
07 Transit motor home
69 VF & 70 VG Valiants needing full restoration
Yamaha Zooma 50cc rocket hopefully going with the Transit.