Hello everyone,

I'm in serious need of some help here. I apologize for the long post, but this is a doozy.

I've got a 2009 F-150 Lariat (V8) that makes a horrid grinding sound. It sounds like nuts and bolts in a blender. It's been happening for about 2-3 years, and i have been attempting to diagnose the issue the entire time. It only occurs when accelerating, and only at very specific acceleration amounts. For example: I can get it to occur at slower speeds if I give it more gas, or at higher speeds with only very slight acceleration. This is very repeatable. When it starts grinding, changing the rate of acceleration in either direction will cause the sound to stop. I've included a video that has the sound (sorry it's hard to hear).

Weirdly, when I haul a massive amount of weight, such as one of our boats or a fully-loaded trailer, this sound disappears entirely, and does not occur at all. But when the weight is removed, this sound returns.

I've taken it to about 6 shops, including both a transmission shop and dealership. Not even the dealership can identify the issue. It only occurs on the road, so diagnosing in the shop on a lift is not possible. They have tried checking vaccuum lines and swapping IWEs and check valve. This was actually done twice. No luck. The sound initially seemed to be coming from the back, but has shifted over time to thr front. Drive shaft also replaced because this is where one shop had seen this issue before. No luck. Done rear diff services and rack and pinion seals, as the sound initially was thought to be in the back. Also new u-joints. No luck.

A shop then ordered decible readers that could be attached to the bottom to determine where the sounds were. Most of the sound was coming from the front end, near the transmission or transfer case. transfer case was evaluated and they found metal shavings in the fluid, similar to where the sound was identified. Swapped transfer case, still no luck.

Took to two different transmission shops. Both agree that a transmission would have gone out a long time ago given how bad the sound is, and both transmission shops agree this is not a transmission issue they've ever seen.

The current thinking is that it could be something like a harmonic, in which something is rattling at just the right frequency in that area, such as the substrate in the catalytic converter that is rattling. But no one really knows, and I cannot keep dumping cash into this thing.

My trucks have never had any accidents and are always well maintained. I'm at wits end with this truck, and I have no idea what else could be causing this issue. Has anyone seen this before?

 

Did anybody try,
Remove the rear drive shaft and drive with only the front driveshaft on ?
Remove the front driveshaft and try the same thing,with only the rear driveshaft working?
There have been cases where the alternator required addl bracing to get rid of vibration and noise problems.
Seems like some are guessing and not having much luck..

Did anybody try to find out why the noise was completely gone when pulling a trailer ?
Does that place more weight on the rear and less on the front , there is a perfect situation that most likely would lead to the cause , but it isn't going to come out and say , Here I am !
It will take some serious investigating and PERSISTENCE.

If the substrate is suspect, the converter can be removed temporarily and a straight pipe installed to eliminate that if it is the problem.
There is no noise that the source cannot be determined , it just takes a little detective work.
Did anybody check for a cracked flywheel flex plate?

 

Hello! Thanks for reaching out. They did remove the front driveshaft to see if its related. It still made the sound when that was performed, essentially ruling out anything in the front related to that. They were not able to deduce why pulling a load would influence the sound, but they surmised that the harmonic or any vibrational frequency would be different. But unfortunately, no one has been able to be definitive.

The straight pipe is a good idea, and I can ask about the flywheel. But I don't believe they checked the flex plate.

 

Had that happen before it was a shield/clamp combo on the exhaust under the driver side. I believe it was on the Y pipe coming off of the driver side exhaust.