Walk-through practices are by definition low-key affairs. The 49ers’ Thursday-morning session, however, had an element of sizzle. “Guys just yapping and talking and alerting each other to everything,” defensive coordinator Robert Saleh said. To Saleh, it was the sound of cohesion. The chatter was the result of a defense that was besieged by injuries in the first half of the season but that finally has enjoyed a measure of continuity – and with it success – over the last month. The team is allowing 3.78 yards per carry, which is more than a yard below last year’s average and one that ranks seventh in the NFL. No opposing rushing attack has had more than 100 yards against San Francisco since Nov. 12. In that way, Saleh and his staff feel very good about their defensive middle going into next season. The next step is sharpening the edges. When it comes to the pass rush, the statistics aren’t nearly as promising. Only seven teams have fewer sacks than San Francisco heading into Sunday’s finale. The 49ers’ sack leader, Elvis Dumervil, has just 5 1/2. If that ends up being the team high, it would be the most paltry regular-season high since Fred Dean had 3 1/2 during a 1982 season that was reduced to nine games because of a players’ strike.