Composition and Chacteristics

• PVC is a hard, lightweight plastic made from chlorine and carbon. It is highly resistant to fire and oxidative reactions, which cause rust in metal pipes. It is also known as vinyl and, unlike other thermally hardened plastics, it could be softened by reheating. Because of its relatively low cost and ease-of-use, it has replaced copper and cast iron for most residential plumbing applications.
  
  

Rushing Water

• Because PVC is thin and lightweight, it transmits noise well. The noise coming from under the house is water running through your pipes, either coming in through supply lines or running out through waste lines. One option is to wrap the pipes with fiberglass sound batt insulation. But be aware, while it will eliminate the noise coming from underneath the house, wrapping the pipes will not reduce the noise from PVC inside your walls.
  
  

Replacement

• Replacing the PVC beneath your house with cast iron pipes will eliminate the noise too. But replacement is by far the most expensive noise-reducing option. It requires a licensed plumber and will require you to shut off your home's water while the work is done. Also, cast iron is subject to oxidation and will rust over time.
  
  

Rattling

• Water running through pipes often causes the pipes to knock against each other and against the home's framing. The fault often lies with the plumbing contractor who did the plumbing rough-in: He probably didn't secure the pipes to the floor joists securely. Installing pipe straps, which is a relatively simple do-it-yourself job, will eliminate the movement that's causing the rattling noise.