My GE convection oven was very noisy. It has 4 fans, two for convection, one for the electronics and one for general cooling. I pulled it out and first ran it in each cycle to identify the culprit(s) that were making the noise. Above all, be aware that there is electrical power on the unit and you can be killed. This is not something for dummies to do!!!

Next, remove the duct shielding from the fans so you can see them. First try tightening/loosening the mounting screws, then loosen them to be able to insert thin pieces of rubber to cushion the mount. Check the fan blades for accumulated crap on them and clean if necessary, carefully observe the fan while running - if you can carefully push on each mount and get it to run quietly, then adjust that mount. A carefully applied drop of good oil on the motor shaft on each side of bearings may be beneficial and do no harm - 3-in-one is good, mop up any drips!

When you have the fans running quietly, reassemble the duct work and tighten the screws well. Run the fans again to hear the sound level. If somewhat noisy, try dampening the sound by paying your hand on the duct. Try adjusting the screws. Seal the overlapping edges with duct tape and also run a strip of the duct tape up the middle line of the duct work as it will dampen sound.

If your cabinet is large enough, hang sound absorption panels on the interior walls, but don't crowd the oven as that could be a fire hazard.

If you do this carefully and methodically you should be able to reduce the fan noise to almost a whisper. But, again, be careful! The oven will be very heavy and will have 220 volt power on it and you have to be capable of doing this safely.