Squeal noise sounds like a relief valve in continuous bypass mode. If true, your oil must be getting VERY hot, which causes rapid pump wear and internal leakage. This would also explain why it works fine with cold oil, but not hot oil.
Next time you start it from cold, listen to the engine carefully as you raise and lower the brush hog. If the engine labors as the brush hog is being raised, that is normal. If it keeps laboring after it reaches full height, that is not normal. Try lowering the handle just a tad until the engine is not laboring, and yet still keeps the brush hog where you want it.
If this fixes the squeal noise, it should help the no-lift issue somewhat with cooler oil, however pump damage may have already occurred.