Succeeding as much for what it hides in its music as for what it shows, this San Francisco trio has quietly made a pair of the most intriguing records I've heard in the last few years. Recorded in a huge warehouse in the rough area of their city known as Tenderloin, Swell's music reflects its surroundings: big, raw, spacious, and unadorned. It builds upward from sturdy, insistent rhythms--drummer Sean Kirkpatrick's loose-limbed wallop, bassist Monte Vallier filling in some of the cracks. Guitarist/vocalist David Freel, whose nonplussed vocals and gnarled acoustic strumming evoke the work of early Giant Sand, casts the often gentle, hypnotic upper layers. On their recent 41 (American), subtle embellishments make the atmosphere a little thicker; "Don't Give," for example, is laced with a melancholy whistling straight out of some Morricone spaghetti-western sound track. But there are no solos or sonic tricks to serve as distractions, and this spareness forces the listener to concentrate on the sound of the individual instruments and how they fit together. At the same time there's plenty to latch on to and--on close listen--an almost orchestral lushness to be gleaned from their simplicity. Saturday, 10:30 PM, Double Door, 1572 N. Milwaukee; 489-3160.