Was there a recent divorce at your home?

If so, here's a possibility' :

On the first day, he sadly packed his belongings into boxes, crates and suitcases.

On the second day, he had the movers come and collect his things.

On
the third day, he sat down for the last time at their beautiful
dining-room table, by candle-light; he put on some soft background
music, and feasted on a pound of shrimp, a jar of caviar, and a bottle
of wine.

When he'd finished, he went into each and every room and
deposited a few half-eaten shrimps dipped in caviar into the hollow
center of the curtain rods.

He then cleaned up the kitchen and left.

On the fourth day, the wife came back with her new boyfriend, and at first all was bliss.

Then,
slowly, the house began to smell. They tried everything; cleaning,
mopping, and airing-out the place. Vents were checked for dead rodents,
and carpets were steam cleaned.

Air fresheners were hung
everywhere. Exterminators were brought in to set off gas canisters,
during which time the two had to move out for a few days, and in the end
they even paid to replace the expensive wool carpeting.. Nothing
worked!

People stopped coming over to visit. Repairmen refused to work in the house. The maid quit.

Finally,
they couldn't take the stench any longer, and decided they had to move,
but a month later - even though they'd cut their price in half - they
couldn't find a buyer for such a stinky house. Word got out, and
eventually even the local realtors refused to return their calls.

Finally,
unable to wait any longer for a purchaser, they had to borrow a huge
sum of money from the bank to purchase a new place.

Then the ex
called the woman and asked how things were going. She told him the saga
of the rotting house. He listened politely and said that he missed his
old home terribly and would be willing to reduce his divorce settlement
in exchange for having the house. Knowing he could have no idea how bad
the smell really was, she agreed on a price that was only 1/10 th of
what the house had been worth ... but only if he would sign the papers
that very day.

He agreed, and within two hours her lawyers delivered the completed paperwork.

A
week later the woman and her boyfriend stood smiling as they watched
the moving company pack everything to take to their new home . . .

. . . and to spite the ex-husband, they even took the curtain rods!

Jackie, look for an animal removal company. If you are sure it is in the ducts, you could ask a duct cleaning company if they will do this. Good luck!

If you could hear them in the shower drain, here are a couple thoughts.

Heat ducts are not connected to drains. However, drains have vent pipes that go straight up to the roof. Often these are uncovered at the top. A bird or other small critter could get into a vent pipe and die. Eventually the remains would likely fall down into the drain line and be washed away. In any case, if you are using your drains, each drain should have an S trap that fills with water and prevents sewer gases from coming up. So it is unlikely you would smell a dead critter from the drain, unless you have a dry drain. This can happen with floor drains or in unused bathrooms. Make sure there is water in all your traps.

Does your bathroom have a ventilation fan? Where does the exhaust go? Straight out the roof or is there a long pipe run? Critters can get into those vent pipes too. Also dryer vents. My house has a horizontal dryer vent line 15 ft. long that goes out the back wall of the house. I had an entire bird nest in there once, with chicks.

Or it may indeed be in the heating ducts. They have cameras they can stick in. Question is, how did it get in? The heating ducts should be a closed loop to your furnace.

A duct cleaning company or critter removal should be able to find it wherever it is.

Just make sure the entry point is located and sealed up if at all possible.

"Carcass", BTW. :-]

Good luck!