No air out of two bottom air-fingers (PS528E)

Hi, Our Middleby PS528E oven has made louder noise for about a month, kinda of gear grinding and engine booming. Each time I turned on Blower only, it makes the same noise. So sounds like something wrong with blower.

The baking had been all fine till yesterday when we noticed pizza dough not adequately baked. Eventually we found no air coming out of two bottom air fingers.

The oven has two air fingers on top, both blowing air. Four at the bottom, two on the right size, two to the left. The two fingers at the bottom left are not blowing out air when blower is ON. I opened the back cover a little bit, can see the motors (two) are running fine.

But why the top fingers get air, not for the bottom ones? Do the top and bottom have separate fans (driven by the same motor)?