In a message dated 8/7/05 9:33:19 PM, j.michael.elliott@GMAIL.COM writes:

<< is there a Known Problem with these things? >>

In my experience, it could be the nest of any one of a number of different
rodents.  The sniff test should give you a clue if that is the problem -- if it
smells like urine or a house with a cat, there's a neat in there.  In order to
clear the debris, I've had some success with hooking up a vacuum cleaner to
the heater box in place of the fiber ducts for the fresh air vents.  Sad to
say, the only surefire way to clean out a heater box is to yank it.

George