I get sewer smell in the basement of my brand new house a couple days a week and I have tried all the usual causes but still can't figure it out. I have a Sand Mound 12 months old.

Using an ejector pump to the septic or gravity? You typically use a lift pump to the mound.

Is there a floor drain in the basement floor ? Any other plumbing fixtures - even just roughed-in or not in use ?

Sand mound, like a sand mound septic field?