New cars are quiet, aren't they? The reason is due to extraordinary efforts on the part of their designers to limit resonance of body structure and sound transfer through openings into the engine compartment. A lot of time is spent on sealing out wind noise and other sources of sound that make the inside of a car something akin to the inside of an oil drum.
Our old cars didn't have the benefit of modern engineering, of course, so they tend to be very noisy inside at highway speeds. If you are in the midst of restoring or otherwise disassembling the car to the point that undersides of panels are accessible, take some time to noise-proof your pride and joy.
Various specialty items are on the market that can help reduce noise in a car, but we prefer to use one of three common products available at home centers and auto parts stores: 1) spray undercoating; 2) Great-Stuff insulating foam; 3) Reflective insulating wrap.
Undercoating helps minimize vibration through sheet metal, so spray the stuff on liberally after painting, inside and outside panel surfaces. It is particularly useful in deadening sound coming through metal air ducts and fender voids.
Great-Stuff (spray, expanding foam) is just that — great stuff! It is extremely effective in filling cavities in car bodies (that's what today's manufacturers do) and holes in firewalls. The only thing to be careful when using it is to allow for expansion. Otherwise, your fender may swell out.
Reflective insulating wrap is usually available in two-foot and four-foot wide rolls. It is particularly useful as a liner in doors, hard tops and over floors. It adheres easily with spray adhesive and is so thin that carpet and other trim don't need to be specially modified. The wrap also has an added advantage that it reflects heat, thus making the inside of some cars less uncomfortable.