It is a about 15 years old Trane xe90, couple of days ago, I noticed that the noise level from furnace is quite higher than usual. Then I go to utility room, and check the furnace, I found that the filter was partially sucked toward the furnace, and quite dirty. And I replaced it right away. Turn on furnace again, it get better, but after a while, there is bad smell from the pipe, I suspicous that the filter was partialy burned, then shutdown the furnace again. Yesterday, the furnace was used for the whole day, right before I went to bed, the strange noisy sound come back, with the bad smell. I am afraid that it will be the notice of something serious wrong, and the noisy is not gone if I only try to turn the heat off from temparature controller, I have to shutdown the electric supply by power down it. This morning, I carefully checked the furnace again, find the symphtom is quite interesting. At first, the flash light show 4 flash ( open high limit device), then after a short while, the ignitor is on, and start the burner, at this time, the upper part of the cover was opened, very strange, no noisy sound this time, and the flash was fast blink ( normal call for heat), but the bad smell still noticeable. Here it is my question. 1. is it possible the open high limit device go bad, but it is, why it didn't shutdown the burner? 2. is it possible the piece of filter was sucked into the burner and cause the bad smell? 3. why it will has the noisy from the furnace, what can be wrong, why the noise will gone when I open the upper cover? 4. is there anything I should do besides change the filter ( I already did it) ? 5. is it dangerous to run the burner like this, or it is better to call for the check up. Really appreciate for anyone has patient to read through the whole story, and wish yours a great holiday.