The gurgle sound may be air in the system. To remove the air you need to cycle all the cylinders from stop to stop. Your hydraulic fluid level could also be low but I believe you said it wasn't. I think air in the system would just make the operation jerky though. The sound you hear may also just be the hydraulic fluid rushing through a stuck relief valve. Since the unit is pretty new it is quite possible that a piece of trash got lodged in a relief valve causing it to leak. If you still have the problem after you cycle the cylinders to remove any trapped air, I would just let have the dealer check it out.
The BH should easily lift the rear of the tractor. You could pick it up and move it side to side but the JD says not to do it (might bend a cylinder rod). About the only reason it would not lift the rear of the tractor is low hydraulic pressure and that is likely due to a bad or stuck open relief valve.
You can check all the obvious things that are less likely to cause your problem like making sure both hose connections are secure, that all the grease points are greased (there are a lot of them on the 48BH), the tractor is fully warmed up, and the engine speed is adequate. The linkages to the control valve could be loose so you could check them also. The BH is made in the US or Canada so the quality is not comparable to the tractor which is made in Japan. On mine the reason it bent a cylinder shaft was that the factory installed a relief valve backwards so it did not work and that was a fairly common problem along with installing the wrong valves.
