You could have it wrapped in insulation to damper some of the noise, but it will be somewhat obscured when there is a cabinet over it. Once there are other appliances running you likely won't notice it. Depending on your cabinet size you could have it made wider to increase air flow/ decrease pressure

it might be because that's all unfinished. tape the grille to the front. tuck a heavy blanket over it and see what that does. if it works, then wrap insulation around it before the cabinet goes in.

(edit: looks like Hallett beat me to the same advice! it's funny when comments come in at the same time.)

I do have space to put a bit of insulation around it... I also have room to redo the sheet metal to make the tunnel wider - thus having more space for airflow and less intense pressure inside. Thoughts?

maybe widening it would work? but try what I said first and see if that does it.

If this is connected to a round supply vent below, you may be able to install a damper. You could then partially close the damper to reduce the air flow.

Thank you! Yes. This is plan B. I appreciate all the comments. They've all been helpful!

At least its a real vent. many times the hvac and cabinet people just allow the air to flow under the cabinet and then escape through a vent cover under the cabinet.