'89 300TE rear suspension noise

My '89 300TE makes a buzzing noise from somewhere near the rear susension whenever I go over dips or rises in the road. I never noticed the noise prior to replacing the hydraulic accumulators, but I
did that soon after buying the car with 180,000 miles on it, so I can't be sure the noise wasn't there before the accumulators were replaced. Anyone know what this is? Could it be the level adjuster? possibly the CV joints or differential? Some other part that rubs against something and vibrates? It sounds like it could be a noisy electric pump or something, but I can't tell - I haven't figured out how to pinpoint where the noise is coming from since the car has to be moving to cause the symptom. Thanks in advance for any insight.
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