Something's in the Crawl Space...or the Wall...

[Chaosjester87]

Originally Posted by UNC4Me

Check where your dryer vents to outside. We had this going on and discovered a bird nesting in the vent.

This is the exact same root cause when I could hear scratching inside my walls. The noise carries way more than you expect. You can buy a little grate cover thing that you put over the vent at any home improvement store.

[annabanana123]

Ours got in the air return near our HVAC unit. The screen had come loose. When it finally died in the wall and the smell permeated the entire house (gross) we had someone come out to figure out where it was. Turned out an entire family of possums had gotten in there and died. Felt bad for them but talk about the worst smell EVER.