Andrew Waxman

2011 f150 fx4 5.0 v8 149,000 miles
I've read through a couple of forums after hearing what sounds like a chain rattling under my hood when accerlating below 2000 rpm, the 2 most common problems being the chain tensioner or a vacuum leak in the 4 wheel drive. I got it vaccum tested at a shop after discovering the sound went away when the 4wheel drive was engaged. He manually checked the solenoid and the right side line had no problem, the left has a very small leak but would not drop below 10.5. he then used a mechanics stethoscope and found the noise farther back leading him to believe its the chain tensioner going slack. My question is would the rattle of a chain tensioner going bad stop when the 4 wheel is engaged? Or could it be something else?

DNA Dan

It's my understanding that a slack timing chain/phaser wear issues is strictly heard on startup. Once the chain is engaged with the engine running, the sound goes away.

Think about the engine running in park or neutral, shifting to 4wd does not change anything. I would say this is a drivetrain issue, IWE, transfer case, tranny, rear end, etc. Start there and see if it goes away.

Andrew Waxman

It definitely doesnt happen on start up or while in netural even if I slowly rev the engine to just under 2000 rpm it only starts when I'm driving in gear and am accelerating, if I'm coasting it sounds fine when I engage 4high and drive the engine sounds perfect which lead me to believe it was the IWE system (if that is the vaccum system used to engage the hubs?) But having a mechanic tell me it's good has me looking for another issue, is there anyway i can self test the trans, transfer case, etc on my own? Even if its enough to get an idea?

DNA Dan

Pretty sure if you engage the 4wd and the sound goes away the issue is an IWE. When NOT engaged, there should be a vacuum keeping the hub separated. Once you engage it, the vacuum is turned off. Seek another mechanic that knows how the system works. If you're low on the left side, you're getting a partial engagement.

Andrew Waxman

Dan from what I read I agree but my automotive knowledge is just so-so, that and I don't want to spend 150$ a visit to keep getting things tested which is why I'm trying to narrow it down.

This may be a dumb question but why would the IWE grind only when the truck is under load/acceleration? Would the engine not generate vaccum in netural or when the truck is in motion? (Like accelerating downhill)

DNA Dan

Any reputable mechanic should be able to test your IWEs easily. They could also pull a tire, remove the rotor and look at it directly. If this happens only under acceleration, this could be a CV joint, U-joint or some other drivetrain problem. You need to look at the univeral type joints in the front end. Do you have the ability to jack it up with both wheels off the ground and turn the wheels? Is it a "clicking" type sound? "metal on metal" sound? Try and post a sound clip.