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Discussion Starter #1
I have a very annoying vibration coming from the engine compartment ONLY between 3700 and 4200 RPM. I have spent a few hours under the car making sure nothing is rubbing or close to rubbing, nothing loose, removed the exhaust manifold heat shields, removed all the metal shielding underneath, and it is still there. It does not matter if it is cold or hot either. And it is totally repeatable every time. Has anyone ever encountered this? Note: I still need to pull the valve cover and check the cams but it seems unlikely that the noise would go away. HELP! :frown2:
 

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Does this occur in every gear?

At rest?

Note that sounds in these cars travel and fool us as to their sources.
 

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Discussion Starter #3
It happens in all gears and in neutral if I rev it up. Like I said, it is totally repeatable while driving and parked. It does not seem to be "bad", just annoying and embarrassing. I have spent maybe 8 hours on it and totally frustrated! Thanks for any help!
 

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I would get a stethoscope made for car stuff and have someone keep the revs in the range while you touch, and listen to, everything.

Some ppl stick a screwdriver near their ears, or dowels.

Exhaust system noises are typically different hot vs. cold, btw.

Wear gloves.

If this doesn't work, I might suggest removing belt (not yours).
 

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Discussion Starter #6
Glen,
thanks for the input but I already checked that. Looks like brand new. I only have 16K miles and everything looks pretty new.... LOL

GLB,
I will try that tomorrow. I have used screwdrives before and they work pretty good. My biggest problem is I don't have someone else to rev it for me as I search.

Keep the ideas coming!
 

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Use vice grip on throttle cable??
 

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InfernoLotus:

"It happens in all gears and in neutral if I rev it up."
So next, push out the clutch and test for it. If the harmonic is still there, your down to engine / mounts. Pulley Bearing? Belt tension-er?
Sometimes those el-cheapo stethoscopes from HF or an auto parts store can come in handy. Even a length of fuel line hose that you can put up to your ear and probe around with the other end. Don't take any unsafe chances with the back of the car in the air as your doing this but it may come to that with the planks off for access. Take your time and do the easy checks first. Make a plan and follow it. Good luck. I had a vibration under the passenger floor (intermittent) that I never could locate that just went away by itself! These cars are one of a kind, that's for sure!
 

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Discussion Starter #9
This noise happens with the clutch in or out. It is just a RPM related issue. It sounds JUST like a piece of sheet metal vibrating. I would have bet $100 it was the heat shields on the exhaust but no, they are off and it is still happening!

So I will continue to T/S tomorrow but I want to share a funny story. I had a TR-7 when I was in college. Every time I took a right turn I heard a metallic rolling sound and then a thump. I removed the carpet and then an access cover to feel inside the door sill. I found a round tool that was left from the factory inside the door sill cavity. It would roll up the rounded edge of the rocker panel and then fall back on right hand turns. I got spray foam and foamed it in place since I could not get the tool out! Fun to be creative.

BTW, I am an engineer and worked as a propulsion system engineer on the Shuttle from 1984 -2007. I still work at NASA on the next generation launch vehicle, the SLS. I tend to be methodical to a fault, but this issue is stumping me!

Keep the ideas coming, no matter how far out they are. All ideas given full consideration.

Thanks!
 

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Nein Kinder
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@InfernoLotus - If you have a shear panel, check that.

Also pull the trunk carpet and look for the rivets (4 I think) that hold the heat shield against the underside of the clam ... I’ve seen pictures of those rivets rusted away so the heat shield ends up loose and can rattle against the clam.

Any chance it’s a loose baffle in the muffler? I had that once on my Porsche and it sounded like it was coming from somewhere else.

Glen
 

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Discussion Starter #11
Glen,
The muffler is actually where I started. It sounded exactly like a baffle broken loose. I have a BWR Silent Touch Exhaust and it can be taken apart for maintenance. I took it apart and it is a straight through design with no baffles. I even started the car with the muffler removed and the vibration is still there. The cat is still installed on the car but I don't think they have baffles. Does anyone know what is in the cats and if that might be vibrating? The thing that is crazy is that it is so repeatable, always at the same RPMs and always sounds the same.
As for the trunk, I did find 2 gone when I removed the muffler and they were all replaced with small stainless steel bolts.
Keep the ideas coming, I will be back working on it tonight!
Fred
 

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Nein Kinder
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@InfernoLotus - Catalytic converters are typically filled with a honeycomb matrix and no baffles. The honeycomb increases the surface area (where the reaction occurs). When a catalytic converter physically fails, it is usually a section of honeycomb that comes loose at the inlet due to heat, vibration, age or all three. I’ve experienced this failure too ... it sounded like a soda can full of gravel. If you can look into the inlet of the cat, you should be able to see the honeycomb and visually determine if it is still intact. This is one reason Lotus says don’t push start your car - you can dump a big slug of raw fuel into the cat that will overheat the honeycomb when it burns and cause it to physically fail.

Another idea related to the clam: it is held down to the subframe with 4 bolts that go through a stack of sheet metal shims. Are all these bolts tight such that the shim stacks won’t rattle? Last time I had my clam off, I glued my shim stack together.

Glen
 

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Maybe the aluminum undertray? If so loosen all the bolts and push it forward and retighten. Tighten the bolts closest to the front and work backwards to the others to avoid any dips in the undertray.
 

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He already said he'd removed that.

Moreover, on almost any large panel, one starts at the middle and keeps crossing over toward the ends.
 

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Discussion Starter #15
Glen,
Good info on all accounts. I will check the clam tonight and maybe get to the catalytic converter also. Report to follow!

David,
I have all of the undertrays off and even the panel under the fuel tank. There are wires and cables which I did not want to take off so I used towels on each side to prevent vibration. Vibration is still there and no change in volume or tone.
 

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I recently solved a similar extremely annoying and loud rattle. At first it sounded like a broken exhaust hanger so I had every joint re-welded...nope. Then later it started sounding much worse...like something was really loose. Also the sound shifted closer to the firewall. I couldn't tell if it was coming from above me or below. I checked behind my 3rd brake light...nope. I checked my hatch hinges...nope.

Then something caught my eye. The nut holding a plastic box which contains the charcoal filter was about to fall off the stud. I hand tightened it until I could get home and WOW no more noise. Once I got home I torqued it pretty tight (not too tight though because it will crack that box). No more rattle!

I hope that's your problem!
 

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Discussion Starter #17
SOLVED! This perplexing problem turned out to be a loose bolt on the alternator bracket. With the help of a lift we were able to isolate it to the bracket using a stethoscope and found the top (and hardest to see or get to) bolt was a little loose. Why it only vibrated at those RPMs is a mystery but I'm so happy now. Thanks to Craig Newton of Lotus of Orlando for taking the time to pool his resources and isolate it. I can thank him for saving me money on counselling, LOL. The team at Lotus of Orlando ROCKS!

Thank you everyone for all your assistance and ideas. I was able to eliminate a few other noises in the process of solving the major annoyance. Plus I had some great one-on-one time with my baby and learned more about her.

Thanks all!
 
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