Pinpoint Test I : The Front Axle Makes Noise In 2H Mode Only And Generally Under Heavy Throttle: Notes

WARNING: This page is about a different car, the 2015 Ford F-150. However, it is still accessible from the selected car via links, so may be relevant.

Locator views for the vacuum lines and vacuum reservoir are located in Description and Operation. Refer to: Four-Wheel Drive Systems - System Operation and Component Description .

Normal Operation

In 2H mode, the TCCM provides a ground path to the IWE solenoid to supply vacuum to each IWE (disengaging the front hubs from the front axle). During certain conditions such as WOT maneuvers, manifold vacuum is lower than desired. To maintain uninterrupted IWE function in 2H, stored engine vacuum is sourced from the vacuum reservoir. A one-way check valve is implemented between the vacuum reservoir and the IWE solenoid to prevent the stored vacuum from escaping into the manifold during low engine vacuum conditions. For noise-free IWE operation under all conditions, the check valve must be operating correctly.

Possible Sources

Vacuum leaks
TCCM
Transfer case clutch
Front axle assembly
Transfer case
IWE solenoid