Thanks for all the advice. I found what I think is the inducer fan
after opening up my Carrier WeatherMaker 8000 (about 4 years old) and
oiled it with wd40 where I could reach, but it still makes the noise.
Perhaps it is the bad bearings Jonathan Kamens mentions.

BTW, I called Carrier and there is absolutely no technical help of any
kind offered. I can understand that with something so potentially
dangerous they don't dare give out advice to consumers, but I'm
spoiled by computer and software vendors who at least will look up the
symptoms and tell you what they know about a model/version. Carrier
offers nothing but "here's someone to call in your area."