The 'growling' is usually the gears non meshing correctly. Mine was caused by a broken axle. The chair was still working but the wheel with the broken axle, while still working was tilted in at the top. I but motors from ebay and just replace them. Insurance is useless to me. My share is usually more to insurance than cash at ebay.
It sounds like you may have bad bearings in the caster wheels; replace them and see if that solves the problem.
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