"dooOIITT" Noise (lol) from the hydrolics?

[BC Dave]

ok its ID that noise time; there's a fairly loud "dooOIITT" Noise (lol) that happens on an infrequent basis a minute after start-up; when the fluids cold; that seem to be coming from the hydraulic pump; the hydraulic fluid is clean; no debris; should I be worried? Air bubbles? Water? just cold fluid? Ideas appreciated (feel free to share your tractor noises too)

 

[winston1]

Don't know I have ever heard that "dooOITT" noise before. What is the temperature up there? Does it sound like it is pressuring up? Engine pulling down any? No answers, just questions.

 

[BC Dave]

Its usually when its been sitting for a while; (weeks) and on cold start-up; so in the morning from say 26 to 45F (-5 to 10C); hydraulics are cold; its a loud hydraulic "dooOITT" noise; im thinking its air bubbles passing through the hydraulic pump; but why? The engine does not seem to load; its a 3 second noise. and happens usually only once maybe twice.