Couple things:

1) That link you posted is a knock off of a Flowmaster 40 series muffler. If you're after a quality tone, the materials you choose for an exhaust system have more impact than once may think. If you're planning on adding a muffler like this to your X (its more of a hot-rod/muscle car muffler btw) check out Jegs. They have all the major brands for this sort of thing, and of course you can be assured their units aren't knock offs.

2) I would definitely NOT recommend sticking a 40 on your X if your worried about interior noise levels. I did a 3" system with a Flowmaster Super 40 (the slightly more agressive 40 series) on the Jeep we resto'd (4.0L inline 6) and its WAY too loud for an X. Try and look for a muffler specifically tuned for euro exhaust characteristics.

3) If you're happy with the sound you have and its just the issue of resonance, consider building a branch resonator to kill it.

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