I sounds like you are smelling sewer gas. Every sink should have a drain trap. The trap is in the shape of a U or a P underneath the sink. The trap holds water so that it blocks any gas coming into the house. Check every sink for a trap.

All the plumbing pipes in the house are connected to a stack vent. This vent is to allow air to air to enter in the all the drain lines. The stack vent emits sewer gas. To solve this problem the stack vent needs to terminate outside the house. If you look at the roof of a house, you will see PVC pipes ending about a foot above the roof deck. Those are the vent stacks.

If you go in the attic you should see the vent stack rise from the attic floor and up through the roof deck. One of your vent stack might have a leak. PVC pipe is glued together so if the plumber forgot to add the glue or made a bad connection, then the sewer gas could escape into the attic. I would not be surprised that a vent stack in your attic was not connected to your roof because the plumber forgot to tell the roofer to make a hole in the roof deck for it! Go into the attic and see if your sense of smell leads you to an area. You might have to remove insulation to find the unconnected stack vent.

Floor drains also have traps, but sometimes they dry out and sewer gasses can then escape. Laundry rooms and basements are places to look. Just pour a quart of water down each.

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AB, I too was thinking the P-traps were not installed but more that the venting is not done properly. I would hire my own home inspector and have him perform an audit of the entire plumbing and venting systems. Yes, it will cost you some now, but will give the builder an exact blueprint to get it right now. If he brings in his guys and fixes 1-2 things, then 6 mo's later one he did not fix manifest, you're out of warranty and it will cost A LOT MORE THEN. if the builder pulled permit, all of this should have been inspected by your local gov't, so you could call a city inspector as well.

Let us know how you do?

"...I’ve noticed it’s coming from the sink pipes throughout the house and it’s coming out of our air vents. If I turn on every sink and shower for a few minutes the smell goes away."

Here is something else you should check. If a sink trap has a slow water leak, then eventually the water level drops down to a point where the sewer gas can enter through the sink drain. The traps under sinks are easy to check. But traps connected to showers and bath tubs are hidden in ceilings and walls. Feel each exposed trap for a possible leak.

I suggest you first account for every drain trap in the house and confirm that each one was installed and operating properly. You made need to open up the ceiling below the bath tub and shower to check there associated traps. Hopefully it won't come to that.

The smell from the supply vents may be due to the foul air may being pulled in by a return vent. Locate all the return vents and check the nearest drains carefully. The problem may be caused one a drain near a return vent. Once the foul air gets into the duct work it spreads it throughout the whole house.