Hi, I now have a Cobra race back box fitted, which sounds awesome. Unfortunately, at certain revs I’m hearing a loud vibration from the boot area. It only happens around 2500 - 3000rpm under acceleration and 2000 - 2500rpm on overrun. And only when driving, nothing at all when revving while stationary (helpful I know!)
I’ve double checked everything, and there’s plenty of room around the box, so I don’t believe it’s the exhaust itself. I know the exhaust will move around a bit, but as I said, there’s plenty of room and it’s not knocking on anything.
I wondered if it was the heatshield under the boot floor, but that is nice and secure. I did note that it is rivetted directly to the floor though, so wondered if it could be the increase noise causing it to vibrate - could it be worth drilling out the rivets, and refitting with bolts and using grommets to dampen/isolate it? I think it sounds metallic, but could be plastic trim buzzing - I’ve stripped the boot trim out to no avail. I’ve also put some padding in various places in the boot to see if that helped (i.e. cables, petrol cap release etc) and also removed a metal panel in front of the fuel filler and refitted with the padding in place.
Mine does this, exactly the same rpm 2500-3000 after changing exhaust, my rattle/buzz is the plastic trim that the roof sits on/in when folded down (only does it with the roof up so not an issue all the time), put something with a bit of weight on the trim and go for a drive, if it stops there’s your issue, cant help solve it though as I’ve not sorted it myself yet…
Many thanks for the reply - I managed to reach the same conclusion as you on the way home from our club meet tonight - I accelerated in 5th and the noise was terrible, and I was convinced it was just behind me. I managed to reach through the gap between the headrests (not easy when driving!) and pushed down on the tray/trim and it stopped. Was looking at the trim earlier today wondering how easy it is to remove (haven’t looked at workshop manual yet), but there is a row of clips along the top, so might be possible to get some padding down the back of it.
Will let you know how I get on - I do usually have the roof down, but been a bit cold & wet of late!
Though it is not easy to get to the "rear package trim” (the shelf that the soft top sits on). You have to remove all the plastic trim behind the seats to access it. Not that is hard, but is just a lot of trim to shift. See page 40/41 here: http://www.mellens.net/mazda/Mazda-Miata-2006-2007/interior_trim.pdf
Anyway, here what’s happening. This shelf has a middle leg/support, which you can see marked in the picture below (not my pic, found it on the web and marked it). From the factory there is a soft foam glued to the bottom of this leg, but with time it slides towards the rear and then there is nothing between the plastic and the metal. Therefore the annoying rattle.Â
Â
Some people solve the rattling issues by forcing the shelf down by inserting some foam/rubber/pipe insulator between the upper trim and the shelf - there is a 15-20mm space.
Mine does exactly the same since i lowered it and fitted a IL Motorsport back box, i found the rattle by having the passenger press down on the parcel shelf which stopped the rattle, it’s a job i’ve yet to sort, but because i know what it is it dosen’t bother me too much.
Booger, I had all the trim out a month ago to replace the roof drains using that mellens pdf! I might see if there’s enough room around the petrol release gaiter to poke some foam under the support leg with a long screwdriver or something. Will let you know if I sort it, but just knowing what it probably is will make it easier to live with
Thanks for the link, I will give that a try if my fix fails (so far so good though):
Remove boot trims (front, back and nearside)
Remove metal panel in front of fuel filler pipes
Locate larger hole in bulkhead above filler pipes where they meet the top of the tank (circled in red). Also note â– â– â– â– sitting on top of tank and remember to use your trombone brush next time you clean the car :
I put an LED torch in the recess behind the fuel filler pipes so that I had a small amount of light to look through the hole and locate the leg of the shelf (not a massive amount of light, but could see the leg in the middle of the car and mine has some white writing on the back which may help to locate it if yours does):
Then wrap a thin plastic flexible garden cane or similar with a small amount of wadding wrapped tightly around the end, tapered slightly, and gently push through the hole and slowly push towards the bottom and the middle of the plastic leg. When contact is made, continue to push and look through the rear window and hopefully you will then see the shelf lift a little. When this happens push a small amount further and then stop and gently withdraw the stick. Hopefully the wadding will then be wedged between the leg and the chassis:
Went for a drive and touch wood, no vibration
My original idea of using the fuel filler release gaiter was a non starter as there is hardly any access.
Afterthought - wrapping a piece of insulation tape around the wadding on the cane before pushing it through the hole might make things easier and the wadding less likely to fall off the cane. Maybe.
Planning to try these solutions at some point to see if it helps. Just wanting to check though, is the resonance sound coming from somewhere quite central in the rear shelf? I can hear vibration more behind the passenger seat and the above fix appears to be something more central. Just wondering if i may have a different area to look at to fix the resonance.
Also I can hear the vibration when revving while stationary for a short moment and pressing on the shelf doesn’t seem to stop it.
The bad resonance/vibration on mine was caused by the central support for the parcel shelf (see picture posted by macadam) and pushing down in the middle did stop it.
Turns out it was the metal plate just to the right of the central pillar that was causing it. It hadn’t been screwed down properly and was causing it to resonate. After screwing it down, taping down any cables and padding out the central pillar to be sure it was much better. =)
Thanks to the sharing of information on this forum, I’ve started experimenting with variations of g40steve’s 69p modification. Foam pipe insulation is one of the most useful items to have in your garage!
Encouraging success rate so far; damping down the remaining vibrations now with finer tuning.
In addition to the lip of the parcel shelf and the rear bulkhead, I’ve damped the perimeter of the shelf, the interior trim around the seat-belt mechanisms and the boot trim where it meets the bulkhead. All achieved with the hard-top in place and in the dark.
Fitted a cobra race box and have the dreaded noise. Removed all the boot panels and the metal plate. No good. Removed the tail pipes and it was miles worse. Put then back on as far to the rear as possible and seems better but having read this thread I’ll probably remove the panels and check the parcel shelf support. Thanks for the info fellas.
Solved mine (previous NC, soft top) for about ÂŁ1, stuffed some foam pipe insulation around the edges of that plastic tray, the edge behind the seats/back panel.
Wednesday. Removed all the inner boots panels to make sure nothing wasn’t rattling in there, removed behind the seat panel, put some foam under the central parcel shelf support. Started raining so bunged everything in the boot and the car to come back to later.
Later was Saturday. Discovered I had left the passenger door open since Wednesday so battery was completely absolutely 100% flat. Put everything back and charged the battery, test drive, all good, annoying resonating sound seems to have gone.
