Ken,

Look at your instrument panel on the fuel economy screen which will show if the engine is in V8 or V4. Make note if the engine is in V4 when you hear the rumble. Some models have a tuning valve in the exhaust system to mostly isolate this sound FYI. My truck will rarely run in V4 while towing our 2600RB but in downward slopes it will occasionally run in V4. Even in Tow Haul which I would have liked them to disable for towing. But that would make this feature a defeat device....another discussion.
Next time you hear the rumble sound, tip into the accelerator pedal just slightly and notice the V8/V4 box on your IP to see if the engine moves from V4 to V8 and if the rumble goes away. If it does, you can kill this feature by setting the truck into TAP mode on the shifter to a lower gear which most likely you are running most of the time anyway.

Let us know what you find.

Jim

I once had a rumble similar to what you are describing. In my case it turned out to be the active noise canceling “feature” in the cab of the truck. The noise was coming from the sound system speakers. Very very difficult to find this problem.

I had installed a platform for the dog behind the front seats. It spanned the entire back seat area blocking the speakers in the doors from the microphones in the headliner. The noise canceling system under certain conditions would crank up the noise canceling sig Al to the speakers since the microphones could not sense it. A closed loop runaway audio loop.

cut some vent holes in the platform and all was well.

the audio amplifier was under the back seat. If you pull the power going to the amp it will doable the noise canceling. Then test.

If you find it to be the AFM you can try https://www.summitracing.com/search/...96&keyword=AFM they are a bit costly.

The other alternative based on warranty and your willingness to tear down the top side of the engine is https://www.summitracing.com/search/...-based-engines

Joseph,

The plug in AFM disabling devices will work and will not mess with the OEM calibrations and will not do any harm to the system. However, they cannot be left in the diagnostic port for more than a few days or the battery will end up dead. They put a draw on the electrical system and do not have an auto shut down.

For the AFM hard part delete components, I would not recommend this since there are no control system modifications where the engine will still be commanding V4 where the spark advance will change even though the engine hydraulic actuators have been mechanically disabled. Also if there is a code set, the engine will revert to default cam position (even if it thinks it is in afm) minimum spark advance which could be as much as 15degress removed. Ouch on fuel economy. And the most important item is the camshaft must be replaced to do this right since the cam lobes are different between AFM cylinders and non-AFM cylinders. So its best to not mess with this kit unless the owner is willing to recalibrate with aftermarket tools (rarely done right), void their warranty and is ok with changing the camshaft. And besides, you will lose fuel economy by disabling AFM.

Jim

Jim, I agree. Local PD had the engine replaced in one of their units and Jasper would not honor the warranty unless a AFM delete was used.

I do not know all of the details with the AFM delete, but I do believe it required an ECM recalibration.

It is a known fact that the AFM components do not like idling for extended periods. Also idling patrol vehicles require oil changes much more frequently than an actively driven vehicle (this was figured out by reading the FORD manual on the interceptors).

Another funny thing is the AFM system caused no oil pressure after it was left sitting at the body shop waiting to be repaired from an accident.

Don't want to go off in the weeds but the cam profile is different between AFM and Non-AFM cylinders. Seems the AFM lifter function needs this difference in lash but in the end the valve motion is the same for all cylinders due to the differences in cam profile.

Jim

If you put the shifter in manual 5, the AFM will be disabled. This might help you see it it the AFM making the noise.

This is if it a 6 speed, anymore speeds and it would take a higher gear than 5th to lock AFM out.

Thanks for all the replies. So I did a test yesterday and took the trailer out while watching the fuel economy screen (I put the truck into tow mode). When the noise happened the truck would go from V8 mode to V4 mode so I assume this is related to the AFM system on the truck (doing what it is designed to do). I could fairly consistently get this to happen if I got up to speed and then slowly lifted off the accelerator.

What is the source of the noise? Valve lifter flutter? Something else?
Is this harmful to the truck? Can I keep on driving it this way and just live with it? I'm ok to live with the noise as long as it's not damaging my truck's engine every time I hear it.
I was going to bring it into the dealership but are they just going to say it's the truck doing what it was designed to do?

I also took the truck out of tow mode and tried this. I was not able to get the noise to occur out of tow mode. Any ideas why? Is it because of the different shift points between the two modes?

Ken.

You can select a lower gear in TAP mode (M) to force the truck into the gear it is spending most of its time in towing the 2600RB while in Tow Haul mode. For a 6 speed, this is fifth. For the 8 speed I believe its 6th but you will need to test this out.

The reason the noise is more prevalent is the higher firing angles and load. In V8 the engine fires at 720deg/8 = 90deg. In v4 its 720/4 =180deg so you are hearing these pulses more easily. In addition, the load is higher per cylinder in V4 than in V8 to do similar work. Even if going down a grade, the load (throttle position) will be high to keep the vehicle up to speed due to all the drag from towing. So how does it save on fuel economy? The manifold becomes unthrottled (less vacuum) while only four cylinders are being fueled so there is a savings but most likely not measurable in tow haul with something as heavy as the Imagine. It would be nice if Tow Haul deactivated AFM but this would make it a defeat device and is why its still operational. Unless you hold a lower gear. If your not hearing this offensive noise while not towing, the exhaust tuning valve is most likely ok.
No damage is occurring where I hope this had not been too much info but you asked.

Jim

I also took the truck out of tow mode and tried this. I was not able to get the noise to occur out of tow mode. Any ideas why? Is it because of the different shift points between the two modes?

Ken.

[/QUOTE]

Ken,

It could be the change in shift points or the actual gear its in but I would need to be in the vehicle to give you a definitive answer. You should be using Tow Haul or the transmission will become busy and will raise transmission temperature. The only time I do not use Tow Haul is while driving in the campground and I want the truck to upshift to reduce RPM and noise.

Select Tow Haul and set your gear selector to M and choose a lower gear once up to speed and AFM will be disabled.

Jim

I experience the "rumble strip" noise frequently but, only under load (with trailer and/or cargo in the bed) and only in tow/haul mode. It occurs in between extended gear shifts. Mostly with light throttle and when the transmission is trying to figure out to go up or down a gear. Dealer service consultant referred to it as "torque converter shark-bite". I have had the torque converter replaced and numerous fluid flushes. The issue remains and I've essentially given up on any dealer remedy... I can, most of the time, "drive through it" by limiting shift indecision as best I can. My transmission is the 6-speed.

Mike

I have a 6 speed with 3.42:1 final drive and the 5.3L and rarely am I out of 5th gear. I have no issues. However, I tow with 89 octane fuel.

Here is what I would suggest without actually looking over your truck.

1) Run ONLY top tier fuel in your truck. You will feel the difference in running top tier fuel if you have been running the discount fuels. I run shell since our Kroger points will transfer there as an example.

2) Run most of the regular grade fuel out and before your next trip, fill with 89 octane. This will allow the engine to learn out any bad stuff (general term to avoid an endless discussion) and will allow the transmission adapts to settle in as well.

3) Move your trans selector to (M) while underway and tap to 5th gear to disable AFM if that's an issue.

Your transmission should almost exclusively stay in 5th gear (D and Tow Haul) unless your traveling at much higher speeds with non towing traffic. I tow at 62-65 depending on wind where RPM ranges from 2100-2800 on hills.

Don't want to go off topic so send me a PM and we can discuss further if you like.

Torque Converter Shudder is the term your dealer was trying to convey where the 6 speed has been good and largely without issue in this regard.

Jim

I have the 8 speed in mine and it is a far better towing experience locking out 7 and 8. Kindof a pain to remember each time you restart the truck to put it in tow mode, then shift to M, then tap up 5 times to get it to read M6. I don't think I heard mine drop into v4 mode once this way in 4000+ miles on the trip we just returned from.