Intermitent noise when driving uphil

I drive a 2008 Ford Expedition and every time I go on long trips after a while on the road the car starts making this grinding noise when driving uphill. As soon as i take my foot of the gas it stops. It has never done it in the city, always when driving on the freeway. I can manage it if I keep the gas pedal as steady as possible but if I try to give it more gas it will start making the noise. This does not happen all the time, it is intermittent. I was told that it might be the 4x4 engaging on the front wheels but I am not sure what it has to do with the speed or gas. I have changed the spark plugs, fuel filter, fuel pump, the dealer could not find a problem with the car when it was at their shop. I checked for loose parts that might be vibrating but could not see any under the car. Can anyone give me some advice as what this could be?

Has anyone checked the fluid level in the differentials? Some 2008 Expedition owners have reported leaking differential seals. If the fluid has leaked out over time, your differential gears will start grinding and making noise. And if the fluid is already gone, a mechanic wouldn’t necessarily notice a leak now.

You have a vacuum leak that is causing partial engagement of the 4 wheel drive on the front wheels. Hard acceleration (going uphill) causes the vacuum to drop naturally. Along with the leak, once the pressure is low enough, the mechanism starts to engage causing the grinding sound. Removing your foot from the gas restores the vacuum and the grinding stops. If your dealer is a Ford dealer and doesn’t know about this then I suggest you go to a Ford dealer that knows how to fix Ford vehicle with four wheel drive.

jesmed1 I will have that checked thanks for the suggestion
bloody_knuckles thanks for the information, I was told this might be the case. The Ford dealer I took the car a few weeks ago said this is not the 4x4 because I would get a message on the dash, which I do not. They recommended a different fuel saying it might be coming from the engine.
Is this something I can figure out by myself? It seems to happen on the left side only, that is where the noise is coming from. Could there be a leak in the left side system only and not something that affects the entire system? You say is normal for the system to lose vacuum on acceleration, why is that?

When it happened in our Ford F150 4x4 (2009) it was the right front only and we did not get any dash lights.

did you just replace the hoses to have it fixed? I still do not understand why is it normal to lose vacuum on acceleration.

Losing vacuum under acceleration is a normal part of the function of an engine. Manifold vacuum is proportional to the unused power of the engine. The less power being generated the more vacuum there is. Under full throttle the engine is producing the most power and this results in lower vacuum. A great explanation can be found on Wikipedia.

I believe in our case the front right hose was replaced and the problem went away.

It is a good thing that the world does not have to deal with vacuum wipers on cars anymore. Yuck, I just made myself feel really old.

I’m not familiar with the automatic 4x4 engagement function, but grinding on uphills or during acceleration makes me think first of a differential problem. Concur w/Jesmed1. Checking the diff fluid levels, front and back, that’s where I’d start.

from what I read so far there is an electric pump that creates the vacuum for the 4x4 system, that is why I was asking about the variations in pressure on acceleration. This coming weekend I will check the oil in the diff and check the system for leaks. Could this be an alternator problem, less power to the pump causing it to malfunction under different conditions? This is intermittent and that makes it very hard to troubleshoot. If I cannot figure out the problem I will try another Ford dealer service shop.

Manifold vacuum is created by the engine. I might be crazy but I have never heard of an engine using an electric pump to create vacuum.

Maybe the OP is referring to an air-locker differential? I think some of those use electric air compressors.

Ford & Dodge diesels use electric vacuum pumps:

@insightful - Diesels do not generate sufficient manifold vacuum and must utilize electric pumps to create vacuum. The OP mentioned changing spark plugs so my safe assumption is that this was not a compression ignition (diesel) engine. My original advice still stands.

bloody_knuckles you are right this is not a diesel engine, is gas. I appreciate you clarifying that for me. I will look into this today.

VOLVO V70: Wipers dependent on throttle position were a PITA. I recall some drivers complaining about electric wipers because electronics were unreliable. What?

so I checked the fluids and everything is good. I checked and there is no pressure loss on the hose at the wheel and no pressure loss on the wheel hub. I am lost I tried everything but could not figure out my problem. Could there be something else with the engine itself? I have no check engine on the dash. Someone suggested the catalytic converter, could that be it? The engine feels like is losing power when this happens.

@bloody_knuckles … just curious, what is the purpose of running a vacuum source to the hubs? Is the vacuum powering some kind of vacuum operated device there? If so, what part is involved that switches the vacuum on and off and operates the device in the hub?

Here is a description of the hub locking system on this vehicle;

Integrated Wheel End (IWE) System
The IWE system contains the following:

•Vacuum reservoir
•IWE solenoid
•IWEs (spring loaded vacuum hubs)
•Vacuum hoses
•Check valve

The IWE system uses vacuum hubs that engage the front wheel hubs to the front halfshafts or disengage the front wheel hubs from the front halfshafts.

The IWE solenoid receives engine vacuum from the vacuum reservoir.

When the 4WD system is in two wheel drive (2WD) mode, the 4X4 control module supplies a ground path to the IWE solenoid to apply vacuum to the IWEs (disengaging the front hubs from the front halfshafts). In 4WD mode, the 4X4 control module does not supply the ground path to the IWE solenoid, vacuum is not applied to the IWEs and an internal spring keeps the front hubs engaged to the front halfshafts.

I see. It’s not for totally locking the differential, it’s for engaging/disengaging the front hubs. Interesting, thanks for the informative post Nevada. If I had that on my Ford truck I wouldn’t have to get out in the mud to engage the front hubs.

I had one of those plastic vacuum reservoirs spring a leak on my old VW Rabbit as I recall, causing some weird symptoms. It was an odd looking gadget, like three tennis balls glued together. Unlikely to be the problem in this case, but should be considered a possibility anyway.