Hydrostatic transmission scream

[JohnDeere8530]

I have a NH combine 2011 CX and upon driving out to the field tonight to see it and there was a hell of a scream/whine coming from the back of it.

It seems loosely connected to the hydro transmission load, but more to engine rpm and starts and stops very very abruptly when it happens - which is most of the time when cutting.

To begin with I thought cavitation, but all the filters are tight and there are no oil leaks etc

As it starts and stops so abruptly it sounds as if it could be something to do with a pressure relief valve or similar?

All my other machines with hydro transmission do make a very similar sound, but only when under acceleration, at 1/5th of the noise level and not all the time

Any pointers anyone ?

TIA

 

[milton-95]

Possibly the alternator, I had a similar noise on a cx 8060, although I noticed that at tickover while hooking on the header, but it came and went too... New alternator and that cured it, bearings were starting to go if I remember right.

 

[Bramble]

Air con compressor worn out??