I'm no expert on hydro, but I believe any restriction in the system that still allows some flow will cause the noise. Of course, some HST whine, or whatever noise you want to call it, is normal. If it's getting worse, maybe you got some trash into the system when you did the maintenance, or did you not change out the filter?
I've had extra noise when the level of Hydro fluid was too low as well. After you refilled the hydro fluid after the change, did you move the attachments around some and then recheck the hydor fluid? This can rob the original level and bring you down too low and cause noise. At least in my experience.
I hope you find it okay, and I'm sure others have more ideas. John