Probably between fan and radiator. Will stop air leaking out instead of going through radiator and will increase distance from fan to radiator .. which often lowers noise level. Case Bottom Spacing Effect on Airflow to Bottom FansThis spacing effect is also often seen with front and top panel/venting too.
Be creative when looking for spacers .. bottle caps, arisol paint can caps, hockey pucks, little brother's building blocks, etc. will all work.Grill & Filter Effect on Airflow & Noise level
Scroll down to see how air flows through round wire grill versus punched metal grill.
Here is a drawing of basic grill mesh designs showing what their open area / airflow area is. Notice hexagonal / honeycomb patterns have much more open area for air to flow through.