Had exact same in one of my early cars, hit a big bump & this big “twang” came out of the speaker from the big spring bouncing around.I had the Motorola Reverberation unit (Vibrasonic) in my first Road Runner. If I remember correctly, I mounted it in the trunk. Might have been under the rear package tray.
This was the under dash control unit.
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I think this was the trunk unit.
View attachment 595505
It was cool sounding as I recall.
Factory wiring diagram was with the unit.The transducers die with age. Just like the old ceramic phonograph cartridges, lack of use is sometimes the killer.
Can you do a visual inspection of the internal?
Yep! wiring diagram with the unit.Did the manual that you have show a wiring diagram?
You seem to have a lot of old school professionals in your area. You could take the unit to a electronic specialist and have them check it out.