Right Rear End AWD GMS Minivan GRUMBLING

I was hearing a humming in the right rear of my 2003 Montan AWD with 200k miles, wrapped up a split in the muffler, replaced a wheel bearing, got new tires, noise is unchanged. It drives fine otherwise.
There is a very slight clunk/click when starting off, so I'm thinking it's the drive train. I've also replaced both rear upper control arm bushings. It's proportionate to vehicle speed (not motor speed), changes slightly when cornering.
Any ideas? Any idea how hard to fix?
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I was hearing a humming in the right rear of my 2003 Montan AWD with 200k miles, wrapped up a split in the muffler, replaced a wheel bearing, got new tires, noise is unchanged. It drives fine otherwise. There is a very slight clunk/click when starting off, so I'm thinking it's the drive train. I've also replaced both rear upper control arm bushings. It's proportionate to vehicle speed (not motor speed), changes slightly when cornering.
Any ideas? Any idea how hard to fix?
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On 1/9/2012 7:10 PM, gmark wrote:

Check oil level in the rear diff, check U joints, check rear input seal (drive shaft) to the diff.
--
I'm never going to grow up.

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