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Are you getting it from both front wheels or just one side?
The odd thing about these hubs and the way they work is, there is vacuum applied to the hubs when in 4x2 and vacuum is removed when in 4x4. I have seen the hubs leak vacuum causing the noise and also the hub itself fail can cause the ratcheting noise while driving.
It should be connected to close the loop.
Just remembered a test to do we do in the shop when we cannot get a helper.
Jack either the right front or left front wheel to where it is off the ground. Start the engine and make sure it is in 4x2. See if you can turn the wheel jacked up without the half shaft moving. Then do the other side.
Check the vacuum canister for leaks and se if there is a check valve in the vacuum lines to the canister. Under load and at high rpms, the engine makes less vacuum, that is why there is a vacuum canister and check valve in the vacuum lines.
Thank you for the update. You are very welcome. Glad you found the problem. Sometimes it turns out to be something small and easy.