Idler pulleys are pretty common on those, it's what keeps the belt tight.

If that spins freely and doesn't have a groove cut into it or other obvious issues there's not much else besides the motor; those are not common but they do develop bearing issues at times and make noise..

Double check that there is nothing on the blower, anything substantial (dryer sheets) stuck to it makes it vibrate loudly.

I usually diagnose by sound, as the different circumference parts spin at different speeds. If you could shoot a video or even audio and post it may be recognizable.

Ok, that is useful. Since this has a removable lower panel you can access it while running. Bad gas valve coils will buzz, sometimes loudly. That's my first guess. Is it heating normally still? No increase in dry times?

Otherwise take something nonconductive (nothing should be electrically hot that you can easily touch but caution beats a bad shock) and start pushing or pulling on things down there and see if you find any part that changes the sound. It sounds low tech and it is but often IDs the problem. I've found coins and objects sitting under motor mounts or in the stamped edges of the base; wires, bullets, and any anything else you can imagine making noises that may come and go as the machine heats and cools.

If it is immediately as ignitor is powered you might start in that area, without hearing it myself the options are wide open.

It's gotta be in the burner system then. Can you get an audio or video?