I’ve had a Cobalt back box fitted to my NC today and on the drive home I noticed that in all gears and at varying revs I am now getting a noise that seems to be coming from the parcel shelf area where the soft top folds down in to. More of a vibration than a rattle…
I’ve found if I leave it in neutral and rev the engine that as the revs die down you get the noise/vibration, but if I do the same but push down very gently on that area you don’t get it… so I’m assuming the new exhaust with different resonance to the stock one is the issue… So, is it something I will have to live with, will it go away as the back box gets ‘bedded in’ or can I do something in that area to sort of sound proof it better….
Not tried it with the top down yet as it’s raining here, but it could be that when the weight of the hood is on the parcel area that it goes away…?
Also, and this is probably just my OCD kicking in, but I’ve noticed that the right exhaust pipe sticks out slightly more than the left pipe…is that normal…
It can obviously only bolt to a fixed point so does it just need the position adjusting on the mounts…. Or is it just how it is…?
Any advice greatly appreciated…
I have one fitted.
The reason one side sticks out more than the other will be down to the fitting on the rubber hangers.
Were the original rubber hangers used?
From memory they are slightly different back/front/side.
I can’t remember exactly which way around.
Defined as a smaller and larger type.
Therefore, IF fitted wrongly that will be the issue, (in my opinion of course).
A stainless steel silencer will expand more when hot and therefore give a different sound resonance.
Likewise, the set up of the exhaust/baffles/wadding etc is obviously going to give a different sound as that’s they whole point.
That is just the nature of the material used.
I “sort of” expected mine to bed in, but again nothing changed.
Difficult to comment on yours as I have other exhaust mods.
Mine is a PRHT and I DID get a vibration where the wind blocker is and put some rubber grommets in to stop it.
Hope this helps.
PS- just found this for you confirming.
Genuine Mazda replacement exhaust mounting rubber. There are two different types on the Mk3. The right hand rear, left hand front and centre pipe rubbers are the smaller type (SI4802) and the right hand front and left hand rear are the larger type (SI4999). Please click on the photo to the right and hit next for a clearer view of their dimensions and exactly where they are situated.
Thanks for this, yes the original rubber hangers were used and I did wonder if maybe they had been put back on in the wrong order as I did remember reading they are not the same on each corner.
I checked this when I got home and they look to be right but I’ll double check tomorrow…
I did also wonder if using new rubber hangers would have been the best thing to do as the original ones are getting on for 13 years old…
Like you say, maybe the resonance will settle down, alter slightly, as it gets used more, time will tell…
Some foam pipe lagging (B&Q) stuffed under the edge of that plastic tray cured a vibration on my NC soft top.
Top tip Mick, definitely give that a try
As Mick said its resonance of the plastic internal roof tray. Common with after market sporty silencers. To be sure find a gear & rpm where it buzzes most then reach behind with your left hand (or get a passenger to do it) and push down on the tray, it should stop. Easy fix with a bit of high density foam in the right area.
Same on mine. Might check mine again.
I liberally used red rubber grease on my original rubber hangers. Perhaps one is sliding more than the other - although as yours is similar it might be a thing with them.
You can only see it from the side really. If you look hard enough. Judge the sound after 1-2000 miles.
Had exactly the same issue with my previous ZSport NC,
The main cause is the support for the “parcel” shelf.
One way to diagnose it is to try it with the hood down as it places weight on the support. If the noise disappears, then that’s the issue.
It takes a bit of doing to get to the support as the trim behind the seats needs to be removed.
This is the shelf support.
The cure is to place a piece of foam under the support to stop the noise.
Hope that helps.
Oh thanks for this, will certainly have a look at it
So I’ve been and checked under the car and the right rubber hangers are being used in the right place so I’m happy it’s been put on correctly.
I can only think that perhaps one of the mount points on the back box itself is slightly out of position…
Im guessing they can only hang where they hang and you can’t really adjust them much once bolted to the rest of the exhaust system…
My offside tailpipe sticks out around 10mm more than the nearside and “toes in”. The nearside is square on. This suggests that the tailpipes were not attached with a laser guided device but by a bloke with a welder.
It’s an exhaust pipe, after all.
And here I’ve just ordered a cobalt! This thread is not filling me with a lot of hope for its fitment or noise!
I wouldn’t change it back to stock. Live in peace with your pipe.
There was nothing wrong with my pipes😁 on the Cobalt and I would have noticed.
It was a few years ago I had one fitted.
The current back box seemed a little off line when refitting it earlier this year. Much better and straight when I swapped the hangers around, they differ slightly, 2 of each type.
Cobalt…
I know I’m being over critical in terms of the exhaust not hanging symmetrical, I just like things to be how I expected especially when paying out of my pocket for them…
As has been said, it’s an exhaust after all, not a precision instrument or something being used by NASA, so the fact it is slightly off, by no more than 10-15mm, I can certainly live with it and won’t be going back to stock, partly because that had to be cut off and isn’t useable anymore…
You can only really tell if you get down to the same level as the tail pipe and compare them against each other so no biggy really, and after all, it’s been bought for the sound more than how it looks…
And I have to say, after a quick blast out this afternoon that I’m really liking it so far… a bit more deeper than stock low down and really sings in the 3-5k rev band, but then once cruising in 5th/6th it settles down nicely and doesn’t drone one bit…
I didn’t get the vibrations I had when the hood was up either, and as I drive it 90% of the time with the hood down it’s not really an issue.
I have put some foam in the tab between the edge of the parcel shelf and the back of the seat trim so that might also be helping…
All in all I’m really pleased with my Cobalt….
Mine fits and lines up fine.
It’s a matter of hanging it and adjusting as the centre pipe is tightened up.
IF you should get one side slightly higher then get a strong cable tie and adjust the other side rubber hanger to suit and line up.
Likewise, you can bend the hanger on the box slightly for back/forward adjustment.
Just be careful to support it and not break the weld.
As above these can only be made to certain tolerances and to be honest are pretty much spot on.
Just an opinion.
So, I emailed Moss with a side on picture from both sides showing the 10-15mm difference in the tips. Really just to see what there reply would be more than anything else, and expecting something along the lines of them being manufactured within certain tolerances etc…
But was pleasantly surprised today to get a reply telling me that they would gladly send me a replacement and then arrange to collect the current back box from me at my convenience… wow, excellent customer service…
The tips not being equal doesn’t really bother me as it’s the sound more than anything that I got the exhaust for, so will see what my local garage would charge to swap them over before making a decision…
Has anybody any experience of the stainless steel back boxes from MX5 City ?.
Don’t go back, I fitted one on mine about a year ago, me and a mate are a couple of inexperienced spanner monkeys but did it in a couple hours. A bit of wiggling around to get it settled straight and level on the rubbers and it was fine. I love the sound it gets better with age, and I’ve not experienced any vibrations, mine is a PRHT.
Hope that helps a bit, cheers
How’s it hanging and does it still cause vibrations?
Just in case there’s any confusion, I mean the Cobalt back box.
