Only on decel? (quiet on accel?)
Does it go away a bit if you drag brake a bit (while pressing accel pedal at the same time?
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Suggestions
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Pull motor. Blow the black dust out of it.
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Check spline coupler, It sounds very dry, Might be even really worn. Apply a dab of assembly paste on it. (See below)
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Replace the rubber disc in the coupler with a new one.
(Something like the one below) -
Check oil in gearbox, add/change oil. (90wt)
