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Discussion Starter #1 (Edited)
I have a 2003 Bonneville. It's my first bike, so I'm not sure if this is normal or not. when I'm traveling around and above 30 mph, there's this hissing sound I can hear when applying throttle. It seems to be in the 2500-5000 rpm range, and is most apparent when accelerating, and especially right after upshifting gears. letting off the throttle, or pulling the clutch in stops the sound immediately, and the faster faster I'm accelerating, the louder the sound is. It also seems susceptible to shock, so if it's hissing and I go over a bump the hissing will be interrupted or change in intensity for a second. It seems to be coming from the right side of the bike, maybe around the carb area but it's hard to tell since I'm traveling fast when it happens.

The bike has the airbox removed, with K&N filters on the carb air intakes. It also has the air injection ports blanked out on the engine. it runs fairly well but tends to backfire when engine braking or letting off the throttle too quickly.

Here's a recording of the sound. I just left my iPhone recording in my pocket on my commute home yesterday. it's most apparent before/after a shift. I had to heavily EQ the file for it to be more audible since the audio was a bit muffled since the phone was in my pocket, so it may sound a little strange, but it's an accurate representation of when it's happening.

Any ideas? what should I check? Is this a potentially serious problem?
 

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I listened a few times and all I am hearing is the beautiful smooth rumble of a Triumph twin.
My guess and that's all it is - you are hearing just the intake sounds from the pod filters. I did not hear any popping on decels either. Sounds to me like it's running quite well !
 

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Discussion Starter #3
Well that's good to hear. The sound I'm referring to is higher pitched, and it definitely could some kind of air intake noise. I know K&N filters can be cleaned, and I think mine may need to be.

The popping and backfiring only happens at much lower speeds, usually when engine braking in first gear like when approaching a stoplight.

anyone else have any thoughts?
 

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On the older bonnies, one source of strange noises was the mechanical speedo cable. Try getting some lube in there to see if it cures/changes...then maybe replace the cable if it bugs you.
Another strange noise source is the Reg/Rectifier mounted under the headlight. Apparently the mounting holes are not always fully lined up and not covered by the bolts...making a whistling sound at speed as air is forced through.

Other things to check...loose/leaking air intake boots. Try spraying loads of Carb Cleaner around the boots (but avoiding the actual open intake filter) while idling the bike...any cleaner being sucked in should change the rpm. Tighten/replace the suspect boot.
 

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I listened a few times and all I am hearing is the beautiful smooth rumble of a Triumph twin.
My guess and that's all it is - you are hearing just the intake sounds from the pod filters. I did not hear any popping on decels either. Sounds to me like it's running quite well !
Same with me, sounds just like mine, sweet and sexy....
 

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Discussion Starter #6
I think that the sound is really just not that apparent in the recording. I'm definitely still noticing it when I ride. It's not part of the normal engine/exhaust sound. I don't think it's a loose part because it only occurs when I'm applying the throttle. I'm thinking maybe air intake or something. I'll see if I can get a better recording somehow.
 

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I think that the sound is really just not that apparent in the recording. I'm definitely still noticing it when I ride. It's not part of the normal engine/exhaust sound. I don't think it's a loose part because it only occurs when I'm applying the throttle. I'm thinking maybe air intake or something. I'll see if I can get a better recording somehow.
Have you made sure the clamps holding the pods on, are tight ? Now they can't be super tight, just until the pod doesn't slip off the intake. Also are the intakes tight enough, what about the carbs themselves ? Check all the small details and sometimes it turns out to be a little problem.....Good luck.

Don't forget there is also noise made when you twist the throttle, induction track noise.
 

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Could it be just the sound of the pods? They will sound louder than the factory designed filter system, which is probably tuned to reduce intake noise (at least we do it in the auto industry).
 

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Discussion Starter #9
I think it’s definitely possible it’s the pods. However, the sound only occurs when the engine is actually under load. Revving while in neutral/clutch in does not produce the sound. Is there more induction pressure when the engine is under load?
 

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Also, have you determined if the sound changes due to road speed or it is just to rpm? Try different gears to the same rpm range to give different road/wind speeds.
 

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Discussion Starter #12
It’s hard to say actually. I suppose I could say that it’s speed related since it tends to only be noticeable above 20-30mph, in second gear and up. I notice it mostly in traffic on the freeway. It continues to make the noise no matter how fast I go, but at a certain point I can’t hear it due to wind noise.
 

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Discussion Starter #13
The main feature is that it’s directly tied to applying the throttle. As soon as I let off the throttle it stops, and it even gets louder the harder I crank the throttle.
 

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Way more air going in the engine under load. Think of how much throttle it takes for 4,000 rpm in neutral compared to in 6th gear.
 
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