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Fan-sounding Knock While Using Rear Air

Greetings. I have a 2002 Mercury Mountaineer. It makes a knocking noise in the rear air system, located somewhere near the back hatch. A few things to note:

--The rear panel has five fan settings: 0, Rear, 1, 2, and 3.

--The front panel with all the front controls on it controls the rear air, so if the front dial for the air type is in the “off” position, the rear air is off; when the front dial for the air type is in the “on” position (such as selecting face-level, foot-level, bi-level, defrost, etc), the rear air can be left off or turned on.

--There is a face setting and a floor setting for the rear air. Changing this setting has no bearing on the knocking, the knocking will either persist or not persist based on some other condition changing.



There are several scenarios that take place, so here they are:

--When the front panel is off, and the rear panel is set to 0, Rear, 1, 2, or 3, the knocking occurs. As I was testing the different fan speeds, the knocking persisted despite the temperature setting. In other words, on cold it knocks, on lukewarm it knocks, and on hot it knocks. It also persists at a constant rate despite the fan speed on the rear air being changed; it knocks at the same rate at 0 as it does at 1, 2, and 3.

--When the front panel is on, and the rear panel is 0 or Rear, the knocking occurs. Similar to the first scenario, the knocking persists despite the temperature setting. In other words, on cold it knocks, on lukewarm it knocks, and on hot it knocks. It also persists at a constant rate despite the fan speed on the rear air being changed; it knocks at the same rate at 0 and at Rear.

--When the front panel is on, and the rear panel is on 1, the knocking occurs only when the temperature is cold. When the temperature is set to lukewarm or hot, the knocking no longer happens.

--When the front panel is on, and the rear panel is on 2, the knocking becomes louder and the rate increases compared to when it was at 1. The noise and rate become worse when it is set to 3, the highest speed. Additionally, it occurs only when the temperature is cold. When the temperature is set to lukewarm or hot, the knocking no longer happens.



That’s the problem, and what all I have tested. I am a little stumped, but my take is that there is at least a problem with the fan (maybe a bearing went out). However, the way it’s playing out it sounds like there could be another problem present, perhaps the A/C. Any thoughts you have are extremely appreciated. Thank you for your time and fielding my question.