There's 'sound', and then there's 'noise'...
I'm not all that keen on the tyre noise in my 156, but the engine (even the 4-cylinder) sounds lovely. The sound in a V6 is in another league. I suggest that any buzzing, droning, rumbing, or clacking noises from the engine are worth investigating, but these should all be apparent when the car is stationary and should be easy to find.
If someone has removed the air intake resonators and fitted a cone filter, I can imagine that would lead to an annoying droning noise at speed - the same applies to modified exhausts.
To get rid of road noise, the easiest fix is 'at source' - the tyres. Get some expensive ones, or the budget-looking type with lots of small tread blocks
When that doesn't work, the boot (as Pascs said) is a likely candidate for sound deadening upgrades, followed by the doors, and then the floor and bulkhead (in that order for the 156, I believe).
Basic theory of sound deadening seems to lie in several modes of operation:
- material to add mass to the steel and change its resonance - bitumen pads, self-adhesive, applied to any steel panels (such as, doorskins, suspension turrets) achieve this. The 156 will already have this material in all key areas. The pads don't have to cover the whole panel - a square in the centre is sufficient, and diminishing returns apply after that. E.g. if you cover the whole doorskin, the door will sound a little more solid when you close it, but only 10-20% better compared to the 80% gained by the factory. Really cheap cars dont have this material, but the 156 is quite well up the spectrum of quality car construction, contrary to what you might believe
- material to trap vibrations/sound waves in a 'sandwich' of soft material. This takes the form of an underfelt-type material but with a bitumen upper (containment) layer to trap the sound. You see this on the bulkhead and under the carpet. The upper layer is very important - just putting soft material in will not help. It needs to be a sandwich. The thicker the sandwich and the more solid the upper layer, the more effective. To upgrade what is fitted, you'll probably need to replace it. But check that the whole floor and rear bulkhead is soundproofed (manufacturers often skimp on soundproofing the rear of the car since most of the mechanical noise is from the front). Generally, all cars have this type of interior bulkhead/underfloor material, but sometimes the engine bay side is a bit thin, the underbonnet pad may be left out of the very cheapest cars, and meanwhile, only the most expensive have the sandwich material behind the door trims.
- material to prevent the transmission of sound through open box sections, including the doors. This, I believe, is the biggest potential gain for the 156. I haven't done my 156 yet, but on most of my other cars, I have 'boxed in' the doors by covering all holes on the metalwork with squares of self-adhesive bitumen sheet. This blocks the holes more effectively than the door trim, and makes the door-mounted speakers sound better (most speakers sound better when boxed). The sills are another prime target for closing holes that otherwise allow sound into the car. The nice thing here is that you can pick up big gains with relatively little material. Why don't the manufacturers stop up all the holes? - well, I guess it would be very fiddly on the production line to cut/apply all the little pieces (e.g. a 10cm square patch for one of the large round holes in the sills).
Something that I am exploring at the moment is expanding polyurethane foam, which Alfa Romeo describe in eLearn as 'thermoexpanding plugs' inserted at the front corners of the sills and windscreen pillars. I know many other manufacturers are using foam in the never-ending quest for rigidity. This could explain why a relatively-lightly built Mazda3 feels so much more rigid than a big old Alfa 164 (or Volvo for that matter). I am in the process of injecting high-density (500kg/m^3) foam into most of the 156's structure, in particular the engine bay side members where they meet the bulkhead, the back part of the inner front wheelarches (which are a box section), sills along the entire length, and all three pillars (legend has it that the rear pillars of a saloon are particularly important for torsional rigidity). All of this has a sound reduction benefit as well, in the third mode of operation I described above, but it is the ride quality benefit that I'm after.
Just my thoughts after about ten years of trying to make cars quieter
I'll tell you what's really annoying - it's when you drive a Kia Rio rental car or something else really cheap and think, damn, this is quieter than my 156...
Low-profile tyres have a lot to answer for.