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I’m running hydrogear ZT3100 transaxles on my zero turn mower.

I know they need to be purged of air to run properly and a sign they have entrapped air is that they are noisy.

I have purged mine following the set procedure twice, first as part of oil change and the second time after a running in period of an hour mow. They are operating fine but a bit ‘whiney’ when getting up to speed after a change in direction.

My question is, what noise do they make when operating in their normal healthy state? Should there be a whine (a bit like a tram car getting up to speed) or should they be silent?

Twice may not be enough, if fluid is at full, I would do the purge again, with wheels off the ground.

I did my HG 2800's @ 50 hours in October & swapped to Full synthetic 20-50. Very quiet. But not totally silent. But much quieter than when I got it New..

I would suggest checking oil levels after next day, you may have External oil expansion bottles. So checking is easy.

But if yours has Internal expansion tanks, after overnight sit, go under & remove the fluid level side plugs & make sure you have just a tiny bit of fluid startng to come out. If so add cap, it's good. If not add proper fluid slowly till it does just begin to overflow. And close the caps. I did mine off the ground & level. Just FWIW.

Thanks for your advice. I do feel the purge worked fine with no air produced on the second run so the first was probably successful. I have expansion tanks on each axle so easy to monitor. I may do as you say and have another go but first I’d like to garner more opinion on what level of noise is normal. Describing a noise is not easy in text, I appreciate!