Strange Sounding Engine

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My engine sounds odd between 1000 to 2000 rpms sometimes. Usually after a few minutes on the freeway. If I barely hit the throttle, like if I'm just rolling around the neighborhood without accelerating ofer 25, it sounds like a turbine. It almost sounds like it's way over-revving, but the rpms are low, and everything else feels normal (shifting, acceleration etc. are smooth), it just sounds odd. It's not related to load or transmission either, because it also does it in park just slowly revving up to 2000. If I keep revving, the sound mostly goes away by the time I hit 3000. Thought it might be something with the aftermarket intake, so I put the old tube back in, but it did not affect the sound. If anything I can hear it more now with the muffling effect of the stock intake tube eliminating the growl I get with the aftermarket.

The only other time I've heard anything close to this sound was years ago, in my old Ranger, 110 degree heat in the desert, it started to overheat when the thermostat failed, and right before the temp hit red, it sounded like this. But again, in this case the temp is fine. It definitely does it more when the engine is warmed up, but I have heard it first thing in the morning too.

For those who read my transmission temp thread, this is different. The sound is similar, but not as high pitched, plus the shifting is normal now.

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Hmm. I was searching for something else, and stumbled across a couple of threads saying it's the fan. I will check that out. Didn't even think to look at the fan when I had the hood up. Is the fan clutch hard to replace? Is it even bad? Sounds like in one of the threads that this was normal, due to hot climate.