I thought I should start a new thread about this because it was going a bit off topic in another thread. So I got the new Mountune cat-back exhaust fitted to my ST a few weeks ago and sometimes notice a knocking sound coming from the back of the car when the road surface has bumps or dips.

This must be the exhaust because I never heard this before it was fitted. I had it fitted at Stoneacre in York and took it back to them so they could check it, and they say it's all ok and just adjusted the heat shield a bit in case it was knocking on that.

From talking to others it looked like a good idea to upgrade the exhaust mounts, which is also something they recommended at Stoneacre. But Mountune disagree, and in emails they have said the exhaust is designed to be used with the stock rubber mounts, and if there is any knocking the exhaust is not fitted properly.

Has anyone else has this kind of problem, even with a different car or exhaust? I'd appreciate any advice :)

All aftermarket exhausts weigh more than the standard item, the OE mounts are softer than upgraded ones, there is only so much ajustment on the exhaust, some people don't have an issue with the OE mounts and some do.

On bumpy roads the OE mounts are going to allow more movement downwards which will then go back up causing the knock on the underside of the car or on the rear beam.

I'd get your RS mounts fitted then road test and adjust afterwards.

What you say makes sense Mark, but that's the opposite of what Mountune are saying. Whether they're just making excuses I don't know, but they also say "There is plenty of scope for adjustment". So will see what the guys at Stoneacre say, but I think they just didn't fit it properly.

Had it back to them yesterday, still not sorted :(. If anything it's worse than before, knocking when going over bumps where it didn't before. On the way home from work today I heard it knock when only the right wheels went over a small dip in the road, probably a manhole, and I definitely heard it knock after the bump so that's proof to me that it can't be anything but the exhaust (would anything else on the car do that?).

Got it booked in again on Thursday and they said a road test will be done, don't know what that will achieve other than proving to them that it is actually knocking...

I just got back from Stoneacre yet again, and now they're saying the next step would be to swap the exhaust for a new one but contacting Mountune first. I might ask them to replace the mounts for new standard rubber ones too as Mountune have recommended against using upgraded mounts. Does anyone know the part number for the standard rubber mounts for the ST (red ones)?

Are you still using the original mounts or the focus RS mounts?

Cant see what changing the exhaust would do they're all made the same.

Mountunes reply that uprated mounts would increase noise and vibration doesnt really apply with the RS mounts which are OE parts and would have passed any NVH tests from Ford. The RS exhaust is heavier so they should work well with the mountune exhaust.

^ Yes they're all made the same but from what I've been told this is the only one that has a problem with knocking. When I called Mountune the other day they were adamant that there is no need to change the mounts, and that could cause other problems. They said they haven't had any complaints of knocking, so I think it's possible this exhaust might have a manufacturing error or something.

I know this sounds stupid but try collapsing the backrests of your rear seats down.. The seat latches sometimes bang on the bar that sits inside of them.

^ I think that's another noise I can hear from the back, but the exhaust knock sounds different, it's lower pitched and "hollow" sounding.

Mine mountune exhaust is doing the same thing and i have changed the rubbers for stiffer ones but made no difference :( did you ever get yours sorted?

^ Yes it was sorted after taking it back multiple times. I'd put the standard mounts back on because they are perfectly fine for this exhaust (that's why Mountune don't include different mounts with it). If it's knocking it simply isn't adjusted properly.