Noise

[deltasierra]

07 E85 Z4. 2.5si Roadster
We expect a crisp exhaust and crackle and pops, with this car hood down it’s loud but fine
Hood up the noise resonates and is really unpleasant, the exhaust looks standard
Can anyone suggest a cure for those days where hood is up.

[enuff_zed]

Double check your exhaust. It could well be that the rear box has been cut open, modified and sealed up again.
Usually this is done on the top surface so hard to see.
If that is the case then you are kind of stuck with it, unless you fit a standard rear box on and lose some of the pops.

[deltasierra]

Hadn’t thought of that, a search found a cutaway of the likely mod cutting the baffles and creating a “straight through” box.
I’ll check it out on the ramp then decide a way forward, thanks for the tip.

[deltasierra]

Can anyone recommend an aftermarket back box that does not resonate with the hood up
I’m thinking an OEM box is rather expensive.

[BumpyZ4]

I have one from Super Sprint installed. Although it's not quite as loud as I would like it to be it has a nice note with no drone.

[Spriteracer]

This doesn't really answer your question, but I get drone in my coupe at motorway speed. I know the exhaust is untouched having had it off the car a couple of times. I have extra sound deadening in the boot above the box, under the boot floor and on the inside if the tunnel + all new hangers. I guess some just do drone.... 70mph in 6th is no fun. It is not so bad at higher revs - i.e. cruising in 5th instead of 6th, but not good for the consumption. I looked at finding a replacement OEM one but have no idea whether it would be the same and could be a very costly experiment. Good luck and let us know if you find a solution.