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When we pull out 10,000lb toy hauler up hills we notice a squeal (like a bearing going out) from under the hood. It seems to be mainly around 3,000 rmps.
It will only do this with the AC if off. If I turn the AC on it almost goes entirely away instantly.
We have had the belt and tensioner replaced as the dealer said that's what was making the noise. "WRONG" It did this before and after the parts were replaced.
Get under your truck and look up between the motor and firewall.
If the firewall is covered you might have a cracked Y-pipe or up-pipe.
Mine is doing the same thing and is going in for a cracked Y-pipe