Gurgling rattle from 1998 ML320?

Can you help me to identify the cause of a strange noise coming from my 1998 ML320 (with 77,000 miles)? The sound occurs when in any gear except park or neutral. Whether or not the brakes are applied seems
to make no difference. I hear it when traveling from zero to 10 miles per hour, when under little or no load (torque). It is kind of a rattling gurgle. When we pull into the garage with windows open it is so loud that we have to raise our voices.
I would also appreciate advice on whether to take it to a local mechanic or long distance to a Mercedes dealer.
Thank you!

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