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Thread: Drone Hum

  1. #1
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    Drone Hum

    Continuing on my rebuild with new muffler and cat.

    I have a loud drone hum all the time. It changes very little with RPM. Sometimes worse when decelerating through 2000RpM.

    Anyone out there know how to stop this? I checked other sites on Drone noise and it gets pretty complicated and expensive to start matching pipe sizes to wavelength estimates.

    Any, the van is almost done and this is killing all the work and money spent on the restoration.
    MT

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    Re: Drone Hum

    If the noise only started once the new exhaust was installed, they pretty much have to be related.
    While loose baffles/shields can produce strange noises, they aren't usually described as "droning".
    I would be looking for a contact point/lack of clearance along the length of the exhaust system.

    Back when we used to build custom exhausts, our rule of thumb was a minimum of a finger's width of clearance.
    Just as important as pipe clearance is to check the welded on hangers, if they are up against a floor board/hanger bracket they can make an awful noise too.

    If you check and find you have plenty of clearance and there are no loose clamps/hardware then its time to check motor mounts.
    LS mounts are notorious separating, allows the engine to move around and can produce a lot of noises.

    I had several strange noises disappear when I replaced both forward engine mounts and the engine is now noticeably higher and more centred.
    Not only was my LS mount separated but it was also compressed a good 1/4in when compared to the new mounts.
    Certainly worth a look

  3. #3
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    Re: Drone Hum

    Thanks BB!

    Motor mounts were replace when engine was removed. So that is out of equation

    Before the new exhaust was installed the van had a bit of muffler hum and changed with RPM. When inspected it had several holes in it all due to rust.
    Again, it sat for 20+ years.

    I went back to the muffler shop yesterday and explained the noise. He was somewhat surprised. He has done this for 30 some odd years and seen some strange things there. But he told me that he has learned many moons ago to replicate the exact length of pipe and distances between all components to eliminate any chance of "de tuning" that would cause drone. All what you said was applied and he offered to add an extra resonator to the system to see if we can at least get it down.

    I picked it up last night and it has improved about 70%. It would take a bunch of resonance work to determine a correct filter drone to get it any better.
    It is not to bad until I de-accellerate through 2000. It sounds like the entire cab is vibrating at about 120 hz.

    The interior has not been install yet and that may add a small amount of dampening but at 120Hz the wave length is so big it is unlikely to expect any more.

    Over the course of the next week or so I will be install Dynamate throughout the van covering the entire floor, inside doors, and over all the doors.

    I also installed Dynamate heat insulation under the engine covers. This was a huge help with all the heat generated from the engine.

    So I guess I will see where I am after sound deadening. The muffler shop has been good and told me to stop in and give a report and will help with what ever he can.

    So, I will go that route. I will report back as I make my way through this.

    Thanks again for the feedback as usual.

    Talk soon

    MT

  4. #4
    Van Addict boogieman's Avatar
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    Re: Drone Hum

    what kind of muffler did you have installed? the aftermarket mufflers are crap, i recently had an exhaust installed with an AP muffler(common aftermarket) and was less thsn thrilled with the sound...i ended up fitting a stock honda accord sanke muffler i got at the junkyard, pretty sure its stainless and the quality is 1000 timee better, vam is quiet. it cost me $14..

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    Re: Drone Hum

    Boogie makes a really good point and if adding a resonator made that much of a difference, then it is more likely to be a resonance vs contact issue.
    You could measure what was installed and we could compare it to OE sizing?
    If it is significantly smaller that might yield some answers?

    I put a Bosal system on our Rav several years ago and while it changed the tone and was a titch raspy, it wasn't un-drivable.
    That muffler was about half the size of factory unit, while I wasn't impressed with the sound quality, OE would have been nearly 3x the cost and wasn't an option at the time.

    What type of system is it?

    If its a clamped system, sometimes an overtightened clamp can squish the pipes and cause leaks.
    Though, those are usually more if a whistle/airy type of sound but in an effort to leave no stone unturned.....
    Note- this is best done with a cold engine/exhaust (less painful) but also needs to be rechecked when hot, just in case.
    -With the engine running, hold your hand near each joint, if its leaking you WILL feel it, but be careful not to burn yourself.

    If its a welded system, make sure you feel over the top side of the weld, its hard to weld what you can't see, I have had a few where I had to oxy-weld a leak shut
    with the engine running (the leak audibly disturbs the flame, until its sealed up)

    Some systems will use flanges and gaskets for joints, sometimes the flanges can be very thin and will deform before applying sufficient clamping force.

  6. #6
    Van Addict boogieman's Avatar
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    Re: Drone Hum

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    heres the 90's accord muffler ( sorry cant remember exactly what year, it had a 90 degree inlet with a brace that i cut off) 2" in/out muffler, its a tight fit but i was able to reuse one of the hangers with a little modification...

  7. #7
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    Re: Drone Hum

    I have run several different custom (radical) exhaust systems as a result of my different turbo and naturally aspirated configurations. I have even run straight no cat no muffler. I have never had exhaust resonance that you describe. I suspect the problem is that the there is no interior, plus a hanger that isn't isolating as it should.

    Did they add a flex pipe? I like adding a flex pipe after the collector. It helps save the exhaust manifold studs, and reduces vibration through the whole system.

  8. #8
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    Re: Drone Hum

    VanCo you are right, modern exhaust systems should include a flex pipe between down pipes and cat, it helps prolong the life of the cat too. At well over 1200 cells vs 800 30 years ago, the metal of the cells is proportionally thinner and, when red hot, cannot handle as much shaking as it used to (dont need to either)
    Also, it is my understanding that a "drone" noise is an harmonic issue due to different parts of the exhaust system displacing gases at different rates, vibrations or rattling are mechanical issues: clearance/contact. I find it easier to deal with a rattle than tuning out an harmonic resonance. That being said, in MyToy's case if the only variable are the cat and muffler I, like boogieman and Burntboot put my money on the muffler. After market muffler are designed for the most part with "performance" in mind: straight thru with a metal or fiberglass packing, OEMs use buffles to deal with sound, that could be enough to make a difference in back-pressure and throw the system out of tune. I would be tempted to go the boogieman way and see (ear) where it goes.
    LG.
    "perfection is achieved, not when there is nothing more to add, but when there is nothing left to take away." A. de St Exupery.

  9. #9
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    Re: Drone Hum

    Yes, of course mufflers change the sound of the exhaust (including drone); however, without the interior, and if the exhaust system isn't completely isolated from the chassis and body you can muffle it to the point of reducing performance and you won't get rid of resonance. The vibration of the exhaust system can resonate through the vehicle via a stiff exhaust hanger. The combustion gasses can resonate through the vehicle because there is no interior to dampen the sound waves.

    I run performance exhaust. The Van likes to breathe. I have found a 2.5" system and flowmasterr series 44 perform best naturally aspirated, but it is a bit loud. To combat this I isolate and insulate well. If the noise bothered me I would go to an OEM style muffler, but I want better performance.

    The big question is, what exact muffler was installed, and is MyToy going for OEM or performance?

    If it is a proper OEM stlye muffler, the hangers are flawless, the interior is installed and insulated, and it still drones as described, my mind will be blown.

  10. #10
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    Re: Drone Hum


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