Box Build *HELP*

[Winter_C 10+ year member]

Ok...so here it goes

Height: 15

Width: 39.5?

Depth: 18

Came up with like 4.9Cubic Feet (without ports and subs) i believe...but here is the thing, i used two 4" PVC pipes 10" deep...i was told this isnt enough and box could whistle...So how do i fix it? The ports and subs both fire towards back of car..the ports are center...Suggestions? and also, with your suggestion can you tell me what my box will be tuned to?

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