I think it's sucking air. Step 1 make sure you have sufficient oil in the system when all of your hydraulics are extended, or check the fluid levels according to the manual which might tell you where the attachments should be positioned when checking the fluid reservoir level.
Hydraulics have a low pressure side (before the pump) and a high pressure side (after the pump) any restriction on the low pressure side will restrict the efficiency of the hydraulic pump, as well as make a change to the sound at the pump and very little change to the engine speed and sound. A restriction on the high pressure side other than a control valve would make the engine stall down significant and possibly cause a leak or blow-out at a weak part of the system. Hydraulic energy is extremely powerful, air compresses, fluids do not compress and the hydraulic power is extremely unforgiving. Listen to your engine actually working once the hydraulics are full of oil, you can hear the engine starting to labour. When your hydraulics are operating at half speed it sounds like your engine is spinning freely and doing little more then burning fuel.
Are your hydraulic attachments on the same pump circuit as your steering or are they on separate pump circuits (different with different tractors) If they are on the same circuit, try turning your power steering all the way over to the steering stops and hold it there for a few seconds, you will hear oil passing through a bypass valve at that point. This will help to bleed air from the hydraulic system if the two systems are common to each other. If they are separate to each other then there might be a similar bypass valve effect when the hydraulics are fully extended, try putting your 3-point lift all the way to the top, maybe that will help.
it's worth a try anyway
Your tractor is very clean, is that because it is brand, brand new or do you power wash it? Power washers are evil things to use on hydraulic components and oil seals, they will force water past the seals and into the oil. Hydraulic and engine oil seals are rarely designed to withstand hydraulic pressure from both sides.
