i deed do the same thing with mi m5 e39 i change the original x-pipe to straight pipe and i lose the deep sound (the sound was like the old american cars) and i lose some back pressure

did you end up find out what size you needed?

no

Guarva, if you will just measure the pipe coming out of the rear of the resonator, that will tell you what size you can use easily. You can get to the pipe by laying down beside the X5 (without getting under it), as the exhaust runs close to the passenger frame rail. You can use a flexible vinyl measuring tape to get the size in mm's or inches, or you can cut a strip of paper about 1/2 wide and 8" long and wrap it around the pipe. Mark where the paper reaches itself around the pipe, then just go measure that piece of paper. Once you have the circumference measurement you can easily find the diameter of the pipe.

So i removed my resonators. And i liked it for a while.

But now the rattling has gotten to much for me. Will the x-pipe fix this problem?