I have a 2007 v6 mustang and im wanting to get dual exhaust on it. I went to an exhaust shop and they told me that the flowmaster 40 series would sound good on it. Does anyone know if this....

It's a matter of opinion. I'm old and like quiet, but to each his own, if you like it, go for it. Fact is it would sound good I have heard the Flowmaster 40 series on a 5.0 and it does sound way cool.....to most folks. Not all.

This video has the sound of the FM 40 www.youtube.com/watch?v=WwOFCGYlZvQ‎

For quiet subdued sound get a take off exhaust pipes and mufflers from a GT (least expensive and least annoying from drone and vibrations). For a moderate level of sound use Dynomax mufflers (has moderate drone and vibrations). For a moderate level of sound use Bullitt mufflers (expensive but best sounding IMHO, has moderate drone and vibration). For a moderate to loud level of sound with bad drone and vibration use Flowmaster or other baffled straight flow type mufflers. All duals on the V6 will drone or vibrate when the idle momentarily dips abount 700 rpm and at 1400-1600 rpm. A true separate dual will vibrate and shake the mufflers badly (and sound like a low-rider). Use an H-pipe like the GT or an X-pipe (the H-pipe does sounds better). To lessen the drone and vibration, use a set (2) 5R3Z-5F240-A damper assemblies and mount them on the leg between the Cats and X-H-pipe, the holes on the straps will have to be elongated because the strap has to be spread to fit a 2 1/2 inch pipe (no mod needed if duals are added further back), once fitted, weld in place. The GT mufflers will already have dampers, other mufflers will need 5R3Z-5F240-B dampers welded to 2 1/2 inch muffler u-bolt clamps attached to the inlet pipe (best appearance). If you use a mid-pipe resonator it will change the sound to "Ricey". There is no room for Heimholtz branch pipes, but a 2-inch pipe crossover where the pipes go over the axle helps kill drone without adding a "Ricey" sound. If you don't mind your interior parts buzzing and rattling and the droning then disregard the aforementioned. I've professionally installed dozens of Mustang V6 dual exhaust conversions and about half complain about said issues. Also, the dual exhaust makes no difference in power, only a premium fuel PCM tune makes a small but noticeable difference, 3.73. gears and Traction-Lok helps too (only with 255 or wider tires).