Locating the Source of a Foul Smell in House?

For the past 4 months I have noticed a rancid dead smell in my house usually when the temperatures are above 90 degrees, the air conditioner is on, and smell is concentrated in the front of the house. I have been hit by this pungent odor all summer. I thought a dead animal in the wall, ducting, or even under house?

After 4 months wouldn't it be dried and decayed by now? My landlord so far has been very conservative, waiting for it to disappear. Could it be the sewer, a nest? I have a lot of squirrels in neighborhood and I've seen rats running along the power lines in my back yard. Please advise.

By Dana B.