Dick Tracy would be proud. The detective work required to unmask the reason for an annoying household sound (where ever it comes from) is usually the most difficult part of the repair process. The worst is over.

The repair suggested by both plumbers is correct. Since the squeak is caused by the pipe rubbing against wood (not uncommon), then the proper fix would be to expand the size of the hole in the wood. This will eliminate the rubbing - and the irritating sound. The cutting should be done with care. Working close to the plastic pipe with a saw could damage the pipe.

An ABS (plastic) pipe-hangar should be installed around the horizontal part of the pipe. And the straight end of the hangar should be nailed to a floor framing member above.

Caution: Don't use metal plumber's tape on ABS sewer pipe. In time it may cut through the ABS. The hangar, which costs about $3, should be placed close to the point where the vertical pipe turns upward through the floor penetration. One hangar at each altered penetration should be sufficient. The strap is installed to hold the pipe in place - not to keep it from moving. Expansion and contraction are normal reactions, and the pipe should be free to move.

Ordinarily, we would recommend using an expanding spray-foam sealant at the pipe penetration as a method of infiltration control (to prevent attic or subarea air from getting into the house). However, in this case, since the foam may also squeak, we fell stuffing fiberglass insulation between the pipe and the wood is better. Air infiltration will be reduced, and the insulation material won't squeak as the pipe expands and contracts normally. And, good luck!