We just built a new house on the exsiting site of the old house with a septic tank. Our ultlity room stinks like sewer almost of the time especially when I am doing laundry or when the air pressure changes. The smell is so bad that I get headaches from this. The laundry is in the basement along with the furnace and hot water tank (both are high effiency). I have contacted the plumber who did our new home and he seems to think everything is fine no lines are cracked, no plugs anywhere and there is a p-trap. I have tried bleach, running water down the lines and nothing is helping. I have been doing some research on line and thinking it could be my front loading washing machine (I clean the drum once a month as indicated from manufactuer)or maybe could it do something with our septic tank or HVAC system? Could the fresh air intake pipe on the outside of the house be to close to the dryer vent as they are side by side? We never had a problem in the old house please help

Sewer odors could be tough search at times to determine the cause. A few questions first. The new house is built on a septic site, but are you using the septic or are you connected to a public sewer? If connected to public sewage, was the foundation of the new house excavated over the old septic or leech field? You say there is "a p-trap"... are there p-traps on all of the fixtures? Most important is this: the plumber who did the work seems to think that everything is fine and this is all that was done? He was on site and knows what work was done and probably would be the best resource to determine if it is plumbing related or a construction issue. Did you contact the contractor who built the house? When you say the house is new, how new is it? How long has this been happening?
My next statement is not intended to offend anybody, but I'm sure it will. Most general contractors and new construction plumbers take short cuts that cause problems down the road. When the problems pop up, they lack the skills to properly diagnose them and propose solutions. Builders pass the buck to the plumber and if the plumber can't figure it out they say "it is what it is..." It all comes down to money.
You had a plumber in the house so I would guess that he checked all of the traps throughout and also ensured that nothing is obstructing the sewer/septic line. The dryer vent should not be drafting sewer gases, but maybe you could put a bucket upside down over the fresh air for a few days to see if the odor subsides. There could also be an obstruction in the sewer.