About ten days ago, my wife told me she noticed a "funny smell" coming from the area of the computer and would I check it out. I did, and could only get a faint odor. A day or two later, I smelled it myself without trying.
My wife claims that I can't smell odors so well, but for me dead rat smell is unmistakeable. I've smelled dead rats in parked cars and, whoooee, some warm days of that and the smell is overpowering.
A few days ago, I noticed the bad smell eminating from the ducts in the forced air furnace. In winter, the use of the forced air furnace is limited because I'm always burning in the wood stove. However, my wife gets up fairly early to go to work and usually I'm still abed then. She turns on the forced air furnace to take the edge of the coolness that has settled into the house overnight. Then when I get up, I burn wood until I go to bed which is usually late. Get up late, go to bed late; makes sense to me.
I know the mechanics of my house very well. I know the entire layout of the plumbing, heating, electrical and so on. So I know the exact routing and layout of the furnace ducts. The heating system vents in two main runs. One is for the two-story side of the house, with ducting between the floors that serves both. The other main run goes out away from the furnace under the middle floor of the house (tri-level design). This run is in a crawlspace. There is a main gallery with laterals that feed out to the sides that go to the registers in the floor. It was from this run that I could smell dead rat.
My first fear was that a rat has somehow penetrated the duct system itself, gotten in there and died. Using a mirror, I looked in several laterals but couldn't see any rat turds or disturbed dust (or dead rats) that would indicate rat traffic. Since I couldn't see in the main gallery, I couldn't check it out. I'd been wondering about this, though. With the furnace turned on, the smell would only come out briefly during initial start-up. If a dead rat was in there, the smell would be constant and quite strong.
With this all in mind, today I went under the middle level to have a look-see. I'd been under there a couple of days ago to set some traps before I did much exploring. This morning, the traps had no takers. So I went on in and started investigating. I know what to look for. Rat turds, disturbed dust, and clawmarks on the framing. I also had a certain lateral in mind to drop down in case I wanted to inspect the main gallery. I went right to that joint and right away found evidence of rat. I started pulling away the plastic coating over the insulation and right away got a strong smell. A little more pulling and I could see a rat tail. Next I went out and got a black trash bag, returned and started pulling the nest apart. The rat hadn't been in there long from the looks of the nest. I put all the contaminated insulation and dead rat in the bag. The smell was very ripe.
I got lucky to find this so quickly.
So, there wasn't any rat inside the ducting. It was right up against the steel ducting at a joint. Even though the ducts are taped at the joints, there are fittings where the steel is just pressed together and apparently the odor was seeping into the system that way. With plastic coated insulation, there wasn't anywhere else for the odor to go. When I first entered the crawl space, there was no odor. It was only when I breached the insulation that the smell came out.
While I was under there, I checked around the whole place and looked at everything. I've had a rat or two get under here before, but I've always discovered this in warm weather after they've left. Cold weather is what drives them inside. Of course, I want to know where they get in. The last time, they had actually tunneled under the foundation footing from outside to gain entrance. Today, I couldn't find the place but it's got to be there somewhere. I sent several more mechanical traps that I will check periodically the rest of the winter.
When I lived in the city, I never had the problem of rats or mice. There, even in a nice but older part of town, cockroaches were the scourge. You can never get rid of all of those; if you eradicated 100% of them on your property, the next night you'd be infested again from adjacent properties. I'll never forget a realtor's comment when we were looking at a home for sale and I found a dead cockroach. Her exact words were, "Oh, that's just an itty-bitty old water bug." Water bug. Lady, I knew cockroaches from Vietnam, those great big fellas, and I know a roach when I see one.
For the many years since that I've lived in a woodsy area, we've never seen a cockroach. We've seen plenty of other pests, though. How about raccoons, opossums, squirrels, rats, mice, carpenter ants, and I don't know what else. They all seem to want to get into your stuff in some way.
I didn't have rats around (to my knowledge, anyway) until about ten years ago. I have some neighbors a ways off who are of a religious sect that believes in storage of food. Well, these people stored up whole grains loose in suitcases in their garage, which they often leave open for long periods. One famous time, they decided to clean out their garage. They got to the suitcases and found that rats had eaten holes in them and infested that part of the structure. When this whole mess was disturbed, the rats fled out the garage door in all directions and since that time, I've started seeing rats from time to time.