Most dictionaries document three versions of the initial sound for whoop: h, w, hw. All three sound fine to me.
I suspect /hup/ is unavoidable if you're going to aspirate the initial consonant. Â In other contexts (what, why, wheel) /hw/ is distinct, but /hwu/ is nearly impossible to say or hear distinctly. Â It's bound to drop back to /hu/ no matter how hard you try to make it clear.
(Edit: I really shouldn't say aspirate here; it's not the same thing. Â But you probably know what I mean.)
According to M-W there are at least six pronunciations of whoop, with three consonant sounds, each with two vowel sounds: wh pronounced as w, h, or hw, and oo pronounced as the u in true or the u in put. At least that's the way I read it.
Peter