Cap'n Kirk- The buzzing of a relief/bypass at low flow-levels is not an unusual trait, and in most cases, it's a nature-of-the-beast. Does the buzzing subside when you throttle it up? If so, attempting to re-machine the valve will yield no positive (and likely some negative) results.
Those valves are typically just a spring pressing on a flat disk, or a piston device. Inside the bore is a hole where high pressure comes in (at the center of the bore), and relief flow exits (somewhere around the periphery, or sometimes through the disk or piston. When pressure comes in, it acts on the high pressure port surface, and eventually, it overcomes the spring enough for fluid to pass to the relief.
The relief pressure is a pretty basic thing... you figure out the spring pressure, and divide that by the high pressure hole's surface area (providing that the piston is capping off this hole... sometimes they use a different, but kinda cagey design)... but when there's enough fluid-pressure acting on enough piston-surface to overcome the spring, you'll get flow.
In a purely hydraulic world, once pressure reaches a certain point, it'll flow pretty constantly... unless you request flow volume (and pressure) to operate your hydraulic accessory... if volume required is high enough, pressure will fall, and the relief/bypass will close.
But in your case, you're in an idling condition, so the bypass is simply allowing flow to return to tank, so you don't get a hydraulic 'stall'. (read up on closed-center and open-center hydraulic systems for more info). When you're idled down, the pump's not going to create a perfectly smooth flow, so the bypass valve will oscillate open-and-shut resulting from a combination of low flow, and also variations due to charge pump's design and flow smoothness- they're not perfect. In this respect, some oscillation at low rates is just nature-of-the-beast.
If it oscillates at HIGHER speeds, it's time to look into the situation. Most times, it's a situation of blocked suction filter (causing cavitation), or an air leak in the suction line (drawing air into your hydraulic system), or a little moisture (becoming steam!), or a worn pump or valve causing goofy operation or cavitation.
Personally, I haven't had one of these Sundstrand hydrostats apart- most everything I work on is either substantially larger, or very small (electrohydraulic, with a 150bar accumulator)... but my guess, and my experience with my hydros, is that you're seeing nature-of-the-beast.
