weird engine noise

'98 Amigo, V6 3.2L 4x4, 5speed, 71,000 miles
Recently I started hearing a new noise from the engine that sounds almost like having an electric motor there instead of the gasoline engine. The
"buzzing / humming" increases with engine speed. It is unrelated to vehicle movement. It is loud enough to be heard at highway speeds. Occasionaly the noise goes away, but it also returns.
Before I start tearing things apart searching for the source, I'd like to hear the group's ideas, or if anyone has had a similar noise.
This stated a day or two after I changed oil and filter, and at the same time the weather got cooler here in Texas. The serpintine belt is about 1 year old.
Any ideas?
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Is the starter disengaging after starting? I had the starter lock into the starting position once on my '97 Rodeo and it made quite a noise. Of course it won't do this for too long before it would burn up, I'd think.
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Yes, it is disengaging.
My original thought was the serpentine belt because the noise would subside some when the engine compartment got warm enough, maybe after driving 10 miles. Then it did a weird thing - it was "quiet" for a while and then started making noise again, while driving down the highway at 60 mph. So now I don't know...
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It might be the clutch assembly on the cooling fan is locking up and that noise is the fan. I am assuming this uses a belt driven fan and not an electic unit.
The viscous coupling on these tend to fail in a condition where they do not disengage.
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Edd, I've stopped the fan with my hand the other day to see if the noise would go away, and it did not. Yes, it is belt driven. Holding the fan still wouldn't eliminate the clutch as a culprit. Right? I think the fan always runs on this engine. I have never seen it stopped while the engine is running.
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This may be caused by the altinator bearings. Try loosening the tension on the belt so the alt. will not turn with engine running. Only run engine for a couple of minutes this way, and if noise is gone, the alt. is the problem.
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