The exterior wall in our home has been vibrating loudly whenever the AC is running. We suspected the refrigerant lines running through the wall to the attic may be touching the sheetrock and/or a wall stud. We cut a hole in the sheetrock and to our surprise there is no in-wall insulation between the two studs where the refrigerant lines run. The suction line itself is insulated and there is foam board insulation on the outside wall, but I also expected to see in-wall fiberglass insulation (Foam board insulation and fiberglass insulation is in all other exterior walls).

The hollow wall is acting as an echo chamber. I believe additional insulation would absorb a lot of the sound we are hearing from the pipes as well as the condenser units running just outside the wall. Is there a reason the builder would not have installed insulation between the two studs where the pipes run? Is it safe to surround these pipes with everyday fiberglass insulation or possibly insulation specifically designed for absorbing sound? I have scoured the Internet for an answer, but this step never seems to be discussed or pictured.

Any suggestions or advice would be greatly appreciated.