Whining/hissing noise coming from near hydro filter. Thinking maybe pump is cavitation not sure how to fix. Machine also try’s to swing a little when first started.
Try posting a Serial Number. When was the last time the hydraulic oil & filters were changed and all the screens removed, inspected, & cleaned.? If you say the machine tries to swing when you hear the noise it could be a partly stuck spool in the control valve that is not centering correctly.
Is it freezing where you're located? I'm guessing you might have contaminated oil. I would try and take a sample off the bottom when cold. Then again when warm. Let the samples set overnight. See if water doesn't drop out to the bottom.
It is a machine a just purchased. Hydro fluid as well as filters were just changed. It has not gotten below freezing yet. CAT0304EVME402384
Was the proper oil used at service? Maybe take a look at the return filter and pay close attention to the bypass. They will sometimes fail at the bend where it is formed. Also be mindful of your pressure cap/valve. If you're not maintaining positive pressure, the pump will pull air. Check the suction pipe to the pump and the flexible joint between the pipes. Make sure all clamps are tight, no evidence of seepage.
What type & grade of hydraulic oil did you put in it.? Did you remove and clean the suction strainer/screen from the bottom of the hydraulic tank while it was drained.? Most important since you just acquired it. Is the Operation & Maintenance Guide in the pocket behind the seat.? If not you need to get hold of one soonest. Cat Publication is SEBU9418 and can be downloaded for $75 from - https://catpublications.com/ EDIT: By any chance did the noise start after the oil was changed.?
If you want a CD at a discounted price try here - https://www.ebay.com/itm/313350896042 These are genuine Cat manuals BTW.