What could be causing that scratching sound coming from my air vent? There is obviously something crawling in my duct work. If you have cats you may not have rats or mice, if the cats are good hunters. But here’s what a cat will do when he’s seeking warmth and comfort in the freezing winter.

In this attic the cats found the warmth of the return air duct and started scratching at the outer plastic cover until they penetrated the duct. This house had two filter grills in the ceiling, one in the family room and one at the other end of the house in the hall by the thermostat and the bedrooms.

When I changed the filter in the grill near the bedrooms I noticed the filter was clean and like new (unlike the other filter) and there was no air sucking the like new filter into the grill. The furnace was on so instantly I knew something was really wrong.

So after gathering up a flashlight and a ladder I ventured into the attic and found the damaged duct (pictured above) and the reason for no air sucking up through the clean filter.

Imagine now for just a moment with that huge hole in the return duct that was once sucking air from the interior of your home. Now it’s sucking air from an attic that is freezing in the winter and hot enough to instantly cause you to sweat in the summer.

If your house attic is well insulated then attic temperatures in the winter will pretty much match the outdoor temperature. That’s the temperature of the air now entering your furnace. Your furnace is trying to heat that cold air coming in through that open duct in the attic.

Crawl spaces under houses tend to stay a little warmer in winter and cooler in the summer. But the air that your house is loosing into the crawl space has to come from somewhere. So now your return duct is sucking air from every possible weak spot in your homes sealed structure. Every window and door or even the dryer vent and kitchen vents when they are not turned on.

That duct in the image above is 16 inches in diameter. That’s a lot of fuel burning that is not going towards warming or cooling your house. Wasted energy!

To compound the problem I knew that if cats could get into the attic, they may have access in the crawl space where all the warm air ducts, or supply ducts are located. I asked the customer and he said he has heard scratching noises coming from the duct work.

I went to a register near where his recliner was located and opened a closed air vent. I got a blast of insulation right in my face. So I knew I had to enter the crawl space. I’ve come to that age where it’s not any fun. Got a screw driver and opened the hatch. Have a look.

This is one warm air duct run under the house. The cats clawed and prodded at the duct work until they broke it down. Then they proceeded to tear it apart and live in luxury among the nicely heated insulation. A soft warm bed. How can you blame them.

As I took a gander around the crawl space it was a total mess. Floor insulation hanging down everywhere. Duct work on the ground and a virtual haven for a cold cat in the winter.

Dead Animals In An HVAC System, Now What?

Talk about disgusting. The amount of damage created by critters in your air duct system can be greatly underestimated.

dead squirrel in furnace
A Poor Dead Squirrel

When a squirrel or even mice and rats get into your duct work they can destroy and damage an entire system. This squirrel found his way into an electric furnace and helped short out some vital parts of the furnace.

This squirrel didn’t know it would cost over $800 for his funeral. There were other repairs not directly related to the inquisitive squirrel but he initiated a series of events that led to the no heat call.

This next customer had no cooling during one of those really intense hot spells last summer. The summer of 2021. When I got there I found the compressor dead. As usual when a heat pump is very old I try my best to sell a new system.

In our conversations they told me about a rat (one of those desert rats) that had gotten into their duct work. The lady of the house had seen it once and they thought it had just gone away. Not so.

You can see the rat in about the middle of this image. Pretty dehydrated and shriveled up in a ball. Funny thing is that he is resting on the filters. This is a very poor arrangement for filters. Difficult to access and replace. I find the easier a filter is to access the more often they will be replaced.

If this customer had replaced his filters in a timely manner he may have saved that compressor. Yes an animal in your duct work can kill a compressor.

How To Get A Snake Out Of Air Vents

If you scream loud enough even a snake will freak out. I remember a personal story that will always be socked away never to be forgotten.

We had a little ranch house about ten miles out of Bend, Oregon in the 70’s. I had just installed a new split system Fraser-Johnston heat pump with a complete duct system.

I was startled by my wife screaming from the bathroom. I ran to the door and it was locked. She was screaming “snake”. By the time we got the door unlocked there she was standing on the vanity/sink freaked out of her mind.

And down on the floor frantically flipping and flopping around the room like a scared rabbit was about a three foot snake. Turned out to be just a bull snake, but in case you don’t know they have every characteristic of a rattle snake, only non venomous and they have no rattles.

She was sitting on the toilet, when she looked down, only a couple of feet away and there, with it’s head sticking out of the heat vent was this snake.

She screamed and I bet if snakes could scream that bull snake would have screamed also. How he got his body through that register, I can only imagine. They can go though some tight spots.

When I investigated the situation, I found a small section of duct not sealed properly to the air handler under the house.

Just kidding about screaming loud to eradicate a snake. I could work, but I only have one incident and one snake to tell about. A real rattler would probably react different. He would likely coil and strike.

Where we live now I’ve dealt with several rattle snakes.


The perimeter of your home is there to keep out the bad weather and intruders, whether they be human, animal or insect. Great care needs to be taken to keep up this barrier.

I had an instance once where pack rats actually climbed the siding of my cabin and chewed through the screen vent into the attic. Another time a rat got in through an open door on one of those hot summer nights. He made a home behind the kitchen cupboards.

My customer with the cat problem is such a loving guy. He has a number of farrow cats. He’s captured most of the adult cats and had them fixed. The humane society refuses to take them. He feels stuck.

His handyman has supposedly plugged the holes into the attic, but in the crawl space he’s afraid to block off the warmth they need for the winter. I’m working with him to find some homes for them.

So, carefully scrutinize the exterior of your home. Properly block, plug or fix any possible access to attics and crawl spaces. Decide what is most important. A healthy secure home, or intruders that will destroy your investment.

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  1. I walked into my basement laundry room and heard a pound rattle noise..(kind of like a very loud cicada in the air duct running the length of the room… it stoped when I made a noise, I closed up the room and left… scared, what could it be? Have had bats a time or too, but did not sound like that… Midwest
    Home in IA

  2. I hear what may be a small animal walking in my ductwork in my basement. I have called out 4 different pest control companies to address the issue. They all say they don’t see any signs of droppings. I have heard this noise for since January. The pest control screened in my gable vents because they had holes in the screen. They sealed up other openings, but didn’t find any evidence of critters. I hear the noise at night and early morning and sometimes during the day. Any suggestions.

    1. Yanni, Did the pest controllers take any duct work apart? Try a heating company. Cut some holes and set some traps maybe.

  3. Mobile home..scratching noises in heater vents which are under the newly replaced floors! Belly pan intact under home..looking down you see the aluminum pans..can’t get to them because of it being a closed system..been here 30 yrs only had baby red squirrel many years ago..any suggestions on how..and what to do? Have 2 cats..lol! Tyvm

    1. Theresa, Sounds like another squirrel. But I really wouldn’t know. It only takes a very small hole in a belly pan to admit one scratching squirrel. If you have AC or a heat pump the refrigeration lines penetrate the belly pan and can leave small holes that allow the critters to crawl up the line set into the belly pan. Exterminator may be the only answer. – Chad

    1. Oh Boy! I’d start with the furnace doors and try to isolate where the cat is and then start taking some duct work apart. Not sure what type of duct work you have but it can be put back together. Good luck

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