I have had several problems with my front hubs and the vacuum system that works the 4 wheel drive and 2 wheel drive system. I have also noticed several posts from others on the forum talking about a noise like rattling or grinding or like a piece of tin that is loose or rattling catalytic converter coming from the front end. Sometimes the dealers cannot figure it out. If the truck is put into 4 wheel drive then the noise would go away. If it was not then the noise would be present. This also includes the loud popping noise that you may here when trying to engage 4 wheel drive and start driving. The hub is not engaging completely because it usually has wear from the actuator rubbing against the iwe gear trying to engage but can't because of weak or no vacuum.
First a little back ground on the 4 wheel drive system in the F150 Raptor. And in fact in all the Ford F150ís. I have also seen this subject on other F150 forums. The Ford 4x4 system for the front axle is a vacuum activated system. Without going into much detail, the system works under vacuum that allows the front hub engage to 4 wheel drive by a part called the vacuum actuator. when the truck is in 2 wheel drive, vacuum from the engine causes the actuator to pull the gear that engages 4 wheel drive in the hub away from the gears to cause deactivation of the 4 wheel drive. When you switch the truck into 4 wheel drive with the in dash switch, it shuts the vacuum off and the actuator causes the gear to engage the hub for 4 wheel drive. This is a simple explanation to get you to understand the system.
The vacuum system is operated with a solenoid that shuts the vacuum off and on. The vacuum tubes that run from the brake booster to the cross over tube that runs between the front hubs is mostly small ľ inch brittle plastic tubing. This tubing is bent and attached to the frame and front upper shock towers. It goes into a dual chamber rubber hose that goes into each hub to activate the vacuum actuator. The plastic tube on the passenger side has an exposed area that is susceptible to cracking or breaking due to heat from the exhaust. The tubing itself is stiff plastic and with age and heat can just crack or break with little effort. It is encased in a plastic sleeve. It has various attachment points across the front cross member in front of the front axle and on the shock towers. What I discovered is that ANY cracks or breaks will cause the vacuum to leak and be weak causing the actuator to still receive some vacuum (maybe 5 lbs) and will pull the engaging vacuum actuator gear for 4 wheel drive away only enough to not be in 4 wheel drive but actually rub and ride slightly on the outside of the hub gear causing all the strange noises everyone hears and rounding off the gears in the iwe and the actuator. The easy way to find out if it is this, is to detach the vacuum hose attachment from the brake booster that runs the vacuum to the vacuum actuators. Make sure you plug the tube on the brake booster so you donít have a vacuum leak there. This can also be the emergency repair to not let the gears rub causing damage to the front hub and gears. The default for this system is to let the gears run together with no vacuum which defaults to 4 wheel drive. The only thing that happens is that the front axles will spin but you will not be in 4 wheel drive because your transfer case will not be engaged unless you turn to the switch on the dash into 4x4. When you eliminate the vacuum you should stop hearing the noise if you have a vacuum leak in the line somewhere.
Since I was having problems with my 2010 truck with 48K miles on it, I decided to go for a more permanent repair and not have to worry about the weak plastic lines running between the two hubs. Plus it was much cheaper than the OEM parts. This will also work on the newer Torsen axle as well since it's the same vacuum system.
I used ľ inch copper tubing approx. 7 feet, 3/16 inch silicon vacuum hose and some poly tubing to build a new vacuum line. Cost approx. $30 at Napa. I used the copper tubing as the vacuum line and the poly tubing as a protective sleeve over the copper for rocks or anything else that may cause damage to the line.
The parts I used. You can see how the poly tubing fits over the copper tubing. This is only an example. My finished piece was approx 7 feet long from Driver to Passenger sides. There was also the silicon tubing that I used in some small areas. Silicon lasts and with stands abuse much better than regular black vacuum hose.
It also makes it much easier to work with as the poly tubing is slippery and allows the copper to be worked with without crimping or bending and causing breaks or smashed copper tubing. I used the same attachment points as the stock tubing. I removed the old plastic tubing and saved it for the venting line that I will talk about in a little bit. I decide to treat each vacuum line and vent line separately. The vacuum line and the vent line are why there is a dual chamber rubber tubing that I mention earlier. The reason will be obvious in a little bit.
I took the down tube from the brake booster reservoir that creates the vacuum and kept it all as one in stock form.
You can see the stock tube in this picture. Itís the one with the rubber 90*
Fitting just to the right of the brake reservoir in the plastic sleeve. (Sorry I donít know how to draw an arrow in a picture image)
This is not one of the problem areas for cracking or breaking and the line works well as is. This line runs down over the driver side upper shock tower into a T fitting that intersects with the vacuum line that runs between the two front hubs.
You can see the T fitting coming down and just behind the large rubber hose. It is going to the dual chamber hose that I wrapped in the larger plastic sleeve. Here you can also see where the copper hose in the poly sleeve is going over the Shock tower and down the frame where the white tie wrap is holding the T to the large rubber tube.
I ran a piece of silicon tubing from the T fitting and attached it to the dual chamber rubber tubing that comes up from the actuator on the back of the hub. Then I built the new copper with poly sleeve that runs from the T and goes over the upper shock mount twisting and turning down and across the frame and front cross member in front of the axles. I then continued it to the passenger side frame up and across the upper shock tower on the passenger side and attached it to the dual chamber vacuum tube that runs to the actuator on the back of the hub.
You can see in this picture the new line running over the shock tower to the dual chamber tube just to the left of the upper control arm running down to the hub. (on the stock oem lines, this is the common area that the plastic line will crack or break.) see the exhaust manifold just a few inches from the vacuum line? Usually there is approx an inch of plastic line that is exposed right there on all 4x4 Ford trucks.
Vacuum line complete. I used my vacuum tool with gauge to check to make sure it held vacuum by attaching it to the tube that connects at the brake booster. Worked fine. I pumped it to 20 lbs and held it for approx 1 minute. It did not drop vacuum at all.
Now I moved to the vent lines for each side. This was pretty easy. I used some of the OEM plastic line that was not cracked etc. ( By the way. When I pulled my stock lines off, I found 2 cracks behind the plastic outter protective sleeve it was in. Both in the area along the front of the cross member. This was what was causing my vacuum leaks.)
I placed one end of the tubing into the smaller tubing chamber on the dual chamber rubber tubing at the hub. I had been seeing on other forums that one of the major problems with Ford 4x4 trucks that have this system was that the actuator diaphragm inside the actuator would be contaminated with water and debris causing the actuator to malfunction and cause holes in the diaphragms rendering the part to be no good and usually a complete hub and actuator replacement. The cause, they think is the vent tubing that in stock form runs just in front of the upper shock mount and terminates. This is a perfect place for water and dirt to enter the tubing since in 4 wheel drive there is no vacuum and can cause debris to enter the diaphragm chamber. So the idea was to run a longer vent tube up into the engine bay on each side. You can see in the photos where I choose to run them.
This is the passenger side. You can see the small plastic covered line running up into the engine bay.
You can see the same line coming up from just to the left of the upper control arm up and under the silver air conditioner tubes and in the next picture you can see I terminated it next to the battery. Much higher and dryer than in stock location.
This is the driver side. You can see the small tube in the plastic sleeve that runs over the top of the brake booster and there was a plastic hole sticking out just above the booster that I stuck the tube into pointing down so nothing can get into the vent tube.
Once I secured the lines etc. to the truck I feel confident that the lines are much more resistant to causing front hub damage.
I hope this helps to solve some problems for others and to understand what that god aweful noise is that is coming from the front end on your trucks WHEN it happens