I used these damping blocks on an old DS214Play on a wooden closet shelf attached to a wall that then was resonating with the drives. Works for that. Obviously doesn't work for fan noise, but I have that set to cool and it hasn't been an issue with overheating:

https://www.amazon.com/dp/B01KAFG8EG?psc=1&ref=ppx_yo2ov_dt_b_product_details

For $8.75, it's worth a try.

Also using these on a DS923+ which is actually quieter with three drives and two fans than the DS214Play with its two drives and single fan.