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Old 09-25-2020, 09:55 AM   #1
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Generator Knock Sound

I am a New owner of Airstream Interstate Ext and am trying to identify the sound that is coming from the Generator while in use. I had not noticed this sound prior to the inspection or while using this over the last few days. There is a Knocking sound that seems to be intermittent while running the generator. has anyone experienced a Knocking Sound It sounds like it is towards the center of the coach but I cant be exact on this. Any suggestions would be helpful.
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Old 09-25-2020, 01:28 PM   #2
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It is most likely coming from the generator exhaust pipe hitting the mounting frame where it passes through . There are rubber vibration mounts that have a bolt through the middle and can also hit the frame and cause noise.
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Old 09-25-2020, 01:54 PM   #3
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Bring it to Cummins Dealer. It's covered under warranty.
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Old 09-26-2020, 11:57 AM   #4
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Our Cummins dealer offered to replace the stock exhaust pipe with a custom-bent pipe that won't hit anything under the chassis, which should make it quieter inside the van. Turns out, Airstream isn't exactly careful about this issue.

The custom pipe would have cost something like $129 plus labor. We didn't have time to wait around for their vendor to make one for us, so we settled for the oil and air filter change, and went on our way. Generator runs fine though it is noisier some times than it is other times, depending on angle of the coach. If we run the generator while driving, it is always noisiest while bringing the van to a stop.

Another thing: the Cummins dealer said no way no how should we ever try wrapping the generator in hush mats or anything else to quiet it down, as this could very easily lead to overheating of the unit. He also said there's no option for a secondary muffler / resonator thingie. So, it's just noisy as all get out.

Last thing: the Cummis dealer said their new quieter variable speed Cummins generator (2.8KW) should fit fine under the van. the cost would be something like $3500 for the unit and another $1000 for labor (again, ballpark, this number might be high). He said that if we ever have to get major service on the generator we have now, it would be smarter to put that money toward the new much quieter generator instead of spending upwards of $1700 if they have to drop the generator to service it and then install it again. So, if that ever happens, that is when we'll cross the bridge and move to the new quieter unit.
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Old 09-26-2020, 02:47 PM   #5
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I have been concerned about heat build-up when I have read about people wrapping it with thick rubber. Cummins/Onan offers (or used to offer) a resonator that helped quiet the sharp rapping noise, though it is still plenty loud.
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Old 09-26-2020, 06:20 PM   #6
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Yeah, I asked several times and pressed them for anything they could do to quiet down the generator, both from the outside and the inside. The only option they offered was the custom-bent exhaust pipe that won't hit the frame the way the stock one sometimes does. Aside from that, it's either tolerate the noise or upgrade to the new quieter generator.

BTW, the Cummins techs at the dealer acknowledged that this particular generator is unpleasantly loud. No kidding, eh?
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Old 09-27-2020, 05:10 AM   #7
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https://shopcummins.com/products/cum...RoCToMQAvD_BwE
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Old 09-27-2020, 07:34 AM   #8
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Generator Knock Sound

I pressed them pretty hard on the muffler question, so I feel confident that if the Cumins techs thought that one would have worked on the generator built into our Interstate, they would have sold it to me and charged for the labor to install it.

Remember, I wasn't at Camping World or an Airstream dealer. I was at Cummins.

That said, please feel free to install one of those on your generator and tell us how it turns out. I would love for you to be right!
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Old 09-27-2020, 08:07 AM   #9
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I think the noise level is about the same but the tone is less annoying with the resonantor installed. The vibration and noise inside is still there so it is more for your neighbors at the campground. I did the work myself so no labor $ and fun to work on the AI.
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Old 09-27-2020, 09:59 AM   #10
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Glad that's working to some degree. The Cummins techs simply shook their heads at me and said there was nothing else that could be done. I would have gladly paid them to cut my exhaust pipe, install the muffler, and put everything back together. (sigh)
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Old 09-27-2020, 10:56 AM   #11
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We just installed the resonator this week. FYI, if you get it from a Cummins dealer it was like $35 and around $57 on Amazon, so sometimes it does pay to get it direct. You can order it online from Cummins and have it shipped to your door for a cost or to your local dealer, too.

It helps a bit. Someone else posted about the decibels and how much sound it cuts down on. I’d just say, don’t expect miracles with it. With that said, still glad to have cut down the sound even if it is only by a bit. Our muffler shop installed it for $25 but they always treat me well so it might be a little more if you take it to a shop, but shouldn’t be a terrible cost to have it installed

The generator is just loud, end of story. Agreed it shouldn’t be wrapped as the heat buildup could be fatal. Wish it was more quite but think there’s just not much we can do but put up with it.
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Old 09-27-2020, 09:17 PM   #12
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Quote:
Originally Posted by GETRIDAONE View Post
I think the noise level is about the same but the tone is less annoying with the resonantor installed. The vibration and noise inside is still there so it is more for your neighbors at the campground. I did the work myself so no labor $ and fun to work on the AI.
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Originally Posted by Hawk-ination View Post
We just installed the resonator this week. FYI, if you get it from a Cummins dealer it was like $35 and around $57 on Amazon, so sometimes it does pay to get it direct. You can order it online from Cummins and have it shipped to your door for a cost or to your local dealer, too.

It helps a bit. Someone else posted about the decibels and how much sound it cuts down on. I’d just say, don’t expect miracles with it. With that said, still glad to have cut down the sound even if it is only by a bit. Our muffler shop installed it for $25 but they always treat me well so it might be a little more if you take it to a shop, but shouldn’t be a terrible cost to have it installed

The generator is just loud, end of story. Agreed it shouldn’t be wrapped as the heat buildup could be fatal. Wish it was more quite but think there’s just not much we can do but put up with it.

I agree with both of the above posts. The resonator is definitely worth it, so long as you have limited expectations. The clatter noise (valves?) is elminated giving the exhaust a slightly deeper and more pleasant sound. If not quieter, then percieved a bit quieter and isn't that the main thing we care about sound.


I will add one more comment about reports of interior generator noise when driving or running the roof ac with the caveat that there may be a difference in the way the generator is isolated on rubber mounts on my Chevy Express-based Avenue versus the Sprinter chassis. But I can barely tell the generator is running from the driver seat if I have my dash ac blowing. And when back in the lounge sitting directly above it and directly beneath the roof ac, the sound or each does not applify the other. In fact, I hear more noise from the ac blower, such that the noise of the generator is not heard. And vibration is practically non-existent from the generator.


Differences between the Express & Sprinter? Is my roof ac just that much noisier since it directly overhead? Did I just get lucky and get a well mounted onan on proper rubber bushings? No way to know for sure, but luckily I have zero complaints from the onan running while inside the van, and pretty happy with exterior noise after adding the resonator.


Now, if AS had just not decided to run the generator to the curb (campsite) side of the van, I'd consider myself totally satisfied.
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