I had a post about this about a month ago, but can't update it due to the timeout for responses.
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Anyway, I did some more checking. It's not the pcv valve, since it's working properly. I checked it with the engine cold and running, then went for a drive until the noise appeared, and checked it again. It's definitely coming from the fan. I also noticed the fan is turning when the truck is started, so it's turning even though there is no need for cooling. Is this normal? When warm, and when the revs go over about 2000, more noise comes from the fan as though it's trying to match the engine speed.
I also don't recall this noise from before I added sound insulation, especially against the interior side of the firewall. It's possible the insulatins masks noises that previously drowned out the fan noise.
Any thoughts on these observations? Thanks