Buzzing furnace and a lot of dust
I have a Bryant Quietline Deluxe furance model 397EAW036100. I have been told it's between 20 to 30 years old. It makes a loud buzzing noise when it kicks on and then the blower will start. The noise continues until the flames stop, but the blower will continue for about a minute or so then stop. It happens 95% of winter. I had the whole thing cleaned and checked out 2 years ago and ask everyone who comes to my home and no one seems to know what's up. They say if it's not broke don't fix it. It's maddening. I had the ducts cleaned out too. They were fine for about a month. I can dust my house and an hour later, the dust is back. I have had it and I am sick of cleaning all the time. Even in the summer when the air is on I get the dust. I replace the filter and now use the 3M 3 month filters. I clean the area where the filter fits and still have dust. Oh, and I tape cheesecloth to the vents (which is ugly) to try to trap dust. Can anyone give me any help? I'd really appreciate it.
***Yes, there is a humifier working on the unit. It is 2 years old installed with new A/C unit, but the noise was present before the unit was installed. There was a humidifier before the new one was installed as well. I am using the 3m pleated 3 month filter. I have not worked on the furnace in anyway other than changing the filter and sucking dust from the bottom of the bed of the furnace. The last professional that was here was 2 years ago.