ETNA — When Fred Schulz and David Couto want to diagnose whatever’s ailing the aged machinery inside the Cedar Creek Grist Mill, they stop what they’re doing and listen.Their favorite, fancy cider press was clunking rhythmically on a recent morning, which meant that mechanical superintendent Schulz had some filing-down to do on the new cutter head he’d installed. This shredder-type press was made in the late 1800s by the Hocking Valley Manufacturing Company, an Ohio agricultural firm, and it’s an impressive piece of engineering — but anything that old is going to need maintenance, Schulz said.Minutes later, Couto manned the mill’s central wheel while keeping an eye on the brightness of a bulb hanging from the ceiling; when it was just so bright but no brighter, Couto knew he was running the wheel at the right speed. But he wondered: “Is this thing growling? Do we need to add some grease?” Schulz came over and the two men tilted careful ears toward the mechanism before determining that it was singing smoothly.“Working with machinery of this era is more art than science,” said Couto, president of the 300-member Friends of the Cedar Creek Grist Mill organization. “You develop a feel for it.” After several months of apprenticeship, that is — and in Couto’s case, one broken drive belt.If you don’t have quite that long, stop by the Cedar Creek Grist Mill on Saturday anyway. You’ll get a feel for running at least some of this artistic machinery — and you’ll learn about all of it — as the mill hosts its annual “cider squeezin’” festival.