Q: What causes the soft howling noise I hear when driving with the AC on? The fix? on 2002 BMW 530i

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With AC on and accelerating conservatively, begin to hear howling at approx 1,500 rpm. Turn off AC with blower speed unchanged, the the noise stops after 2-4 seconds. When it is howling and I have distance to coast to a stop, the pitch of the howling changes as the auto tranny changes gear. No performance probs with AC or otherwise. At 110,000 miles, have new alternator, water pump, belts, mechanical tensioners, belt idler pulley, but noise persists. How to test the compressor clutch? Have seen this reported on other blogs, but no definite fixes given, only possibilities.
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