Dull knocking at highway speeds - Exhaust resonance?

[Badunit]

Does anyone else experience a dull knocking sound (and feel) from the engine/exhaust in the 1600-2000 RPM band that goes away completely at around 2100-2200 RPM? I'm assuming it is an exhaust resonance (just my guess) and wouldn't really consider it an issue other than it matches up with normal highway speeds of about 55-65 MPH so I end up hearing it a lot.

 

[BillGotthelf]

I don't know if its the same noise but around 1800 rpm it sounds like the oil in the motor goes away and returns around 2000 rpm. Just a change in engine sound and thats the best way to describe it.

 

[Badunit]

Sounds similar to what you described.



161 reads of this thread and only one reply, I don't know if that means no one else is hearing the same thing or means that it's normal.

 

[PFreeman]

Hey, Guys! Glad to hear others are hearing the "knock" too. It seems to be RPM specific. I seem to feel it thru the pedals as much as hear it. Guess it's just all that H. P. rattling around in there! I have about 4k on my 2003 3500 H. O. 4wd quad cab, srw short box. Replaced a 2003 FERD powerstroke similarly equipped. Cooked its transmission at 35. 9k pulling a 15k fiver! The new DC handles it like a dream. No downshifting on hills. The 6 speed is really smooth, but VERY noisy under 1400 RPM. Is this typical, or can I expect problems later? Dealer says "normal" what else?

 

[Vaughn MacKenzie]

PFreeman, I assume at 1400 RPM you are cruising unloaded on level ground, in town, and not towing. If you are running 1400 rpm towing you are seriously lugging the engine and it will make some noises in protest. . . because it's not good for the engine or transmission.



Vaughn

 

[lschultz]

Mine does not do what you are describing but from 1900-2100 its got a vibration(not the word for it but best I can do)get over that and its ok.

 

[Rosco]

My truck also "knocks" at lower rpm's. It is really noticable while chugging around town. I was a little worried that something may have been wrong because as I recall my '03 SO didn't seem to make that noise, or at least it wasn't as pronounced.



However, once I get on it, the engine goes into that smooth power-band growl I like so much .



As a matter of fact when I fueled up tonight I added a bottle of Power Service additive to see if it will smooth out the knock. I was only going to add a half bottle but it was dark out and I went a little overboard. By adding the whole bottle it says I will increase my cetane number by 6. I will let you guys know if it helps or not.

 

[FM]

I don't have any knocks when under way. I have the "vibe" from around 2k to 22k that someone here said was a characteristic of the engine. I also have a random intermittent knock when sitting at idle that raises my eyebrow more than anything else.

 

[timjones]

22K RPM Good god man that should sound more like an EXPLOSION!!!

 

[GWDiesel]

IF you think you notice a throbing sound @ around 19/2100 rpms

or the EX sounds loud DO NOT change to another form of ex system, it just has more of It! my 03 is loud with a Donalson

truck muffler that has a baffel with 5" system. GWD

 

[Badunit]

Thanks GWDiesel. That was going to be my next question, whether an upgraded exhaust system would reduce the noise.

 

[PFreeman]

Originally posted by Vaughn MacKenzie

PFreeman, I assume at 1400 RPM you are cruising unloaded on level ground, in town, and not towing. If you are running 1400 rpm towing you are seriously lugging the engine and it will make some noises in protest. . . because it's not good for the engine or transmission.



Vaughn

Vaughn, I always try not to lug the engine, I stay above 2000rpm when pulling. The transmission noise I am experiencing is a grinding rattle, sounds like gear lash, it happens from idle to apx. 1500 rpm in any gear. (sounds like I remember the old school busses trannys when the were badly worn). I have a friend who runs a hotshot delivery business using a 1998 3500 drw in which he has installed a six speed. He says that the noise is just a characteristic of the dodge six speed. I'm not so sure. This is the first manual trans. that I've owned that was noisy. You would think that when spending over $42k for a truck, the transmission would be quieter. All that being said, I wouldn't go back to a powerstroke for anything. LOVE THE DODGE!!!!!

 

[Rosco]

The Power Service (in the silver bottle) has definetly smoothed out the engine "knock" quite a bit. I am going to use it at every fuel fill from now on.

 

[FM]

Tim, you could be on to something there. As the kids say today "my bad". Should read 2. 2k

 

[GLolas]

Forgive my ignorance, but what exactly does this knock sound like? In my 03, the engine noise is much different right of idle during light acceleration than it is later in the "curve". It seems to "rattle" from about idle to maybe 950 RPM and then it goes away as the RPMs go up. Is this normal for this 03 engine? Its an SO with a 47RE and it has about 2700 miles on it.

 

[Rosco]

GLolas, In my post's I am using "knock" to describe what I think to be noise produced by the combustion process. Like I said the Power Service seemed to help some but it definitely "rattles" at lower rpm's. However, once I put my foot in it the noise disappears and I hear the "smooth" Cummins power being made under the hood.

 

[sag2]

Off idle especially cold, mine sounds just like the old engine. One you get the rev's up it quiets right down. It has done it since new and has 14,000 miles now. My co-workers 03 does the same thing.

 

[Badunit]

The noise I'm talking about sounds and feels similar to a connecting rod knocking, but also like a rumbling exhaust. That's about as close a description as I can give.



My engine is smooth off of idle, builds up to this knocking noise starting at about 1600 rpms through 2000 rpms, is smooth as silk at around 2100 rpms (about 70 mph), then around 2300 or 2400 rpm (80 mph) it is a rumbling vibrating beast if I'm on the throttle a little hard or smooth if I'm on it lightly. The knocking at 1600 rpms is there even when I'm off the throttle.



This being my first diesel truck and first Cummins I'm not sure what to expect. Maybe all this is normal? Maybe some more break-in is needed?

 

[wlowe]

Badunit, my truck makes the same sound, not loud, barely noticable. When I first got the truck I thought it might be a rod knocking but the 01's are so noisy it was hard to tell. It doesn't do it at idle, just between 1,400 and 2,000 RPM. I went to the dealer and test drove another truck and it made the same sound so I think I'm safe. The 100,000 mile warranty is there just in case I'm wrong. After 18,000 miles it still is the same, no better, no worse. The engine runs smooth as silk so I'm not going to worry about it.



Bill

 

[Prairie Dog]

Mine had the same knock until it got 4 or 5 k on it. it sounds more like a main bearing kind of knock because it doesnt lag behind throttle let up. I think it's normal, by the time I had 6K on her it was 99% gone.