This doesn't sound right to me. The HST will certainly make noise as you drive it around, but I'd call it more of a "whining" sound. You'd hear that with any HST tractor, any brand. You'd also hear it regardless of the RPMs while moving around. Some could be a bit louder than others I'm sure, and it can get louder if driving with higher RPM. But "squealing" doesn't sound right, especially if I understood you correctly and you only hear it once you hit 75% or greater throttle, whether moving or not.
I would call the noise the relief valve makes a definite "squeal", but you wouldn't be hearing that just running it and moving it around. You'd have to be doing something like continuing to press and hold the loader lever after the bucket has fully dumped or something like that - where the hydro pumps relief valve has to open to relieve pressure until you stop.
Pretty hard to tell unless we can actually hear what you mean by "squealing". I think if, as already suggested, you drive some other HST tractors and you hear the same type of noise it could well be the normal HST sound. Otherwise, seems perhaps there is something a bit off with that particular unit.
