To me that looks like a badly machine built wheel, could be wind-up on the spokes or just where the builder bent them during assembly. Where did you get it from?
Quick test, rest the axle on a block of wood and put your weight on your arms on opposite sides of the rim (be gentler on a road rim, but an mtb rim should take almost your full ‘press up’ weight). If you hear any ‘pings’ then the spokes are badly wound up which means the tension is all over the place.
If it were mine I’d get a truing stand and tension gauge, loosen off all the nipples until the thread just begins to show then bring the tension up gradually truing as you go. The Sheldon Brown website has a good guide to building wheels, just skip past the assembly bit to the truing/tensioning bit. Worth doing as well built wheels feel much nicer and last a lot longer, badly built wheels at best go out of true, worst fall apart.