I had a Bosch fuel pump in a Bug Eye that made twanging noises mainly on downshifts from memory, sounded like something in the back door or the handbrake cable, took weeks to work out what it was but it's a possibility it's your problem also.

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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.