JdolsonSo I saw this post and I was experiencing the exact issue. I have a new 2021 gc1723eb and once warmed up a significantly loud squeal. It would only go away when I moved a hydraulic control lever. I noticed somewhere someone that had this issue found it disappeared after removing and reinstalling backhoe. Also read about hydraulic couplers being incorrectly installed so the 3 point hitch would not raise. So I removed my backhoe...and I also had hydraulic couplers incorrectly installed so my 3 point hitch would not raise. I reconfigured the couplers...3 point then raised. I exercised the 3 point hitch several times up and down. Put backhoe back on. I have used several hours since... no squealing. Is it possible that air was trapped and never removed from the 3 point hitch system causing vibration in the hydraulics ?. The hitch could not have been operated from factory the way the couplers were configured. So far so good. No squealing.
See my April 15th 2020 post - It is my understand a Hydraulic relief valve issue was my tractors problem - the spring was too long causing a higher regulated pressure then designed for and that the tech could not adjust out so the fix was to shorten the spring to get to the proper pressure. In my opinion due to the fact that your hydraulic couplers were configured wrong may have put a higher back pressure on the Hydraulic system causing the squeal and when you reconfigured the couplers the proper pressure was restored Also the wrong coupler configuration may have cause a restriction in the system to cause the noise
Hope this helps - again just my opinion