Downshifting, as in shifting to a lower (larger diameter) gear. Ideally, I shift gears just before I need to so that I'm not pressing hard on the pedals when the chain moves from one gear to another. But on unfamiliar trails, I'll sometimes come around a blind turn and be confronted by a steep climb or a rooty/rocky section of trail. In those instances I have no choice but to shift as I'm pedaling hard. I hate having to do that, since it makes a nasty grinding sound, which can't be good for the chain and gears. Not me personally, but I've had it checked by half a dozen bicycle mechanics over the past few months. Wouldn't they have noticed something like that? No. I only bought the bike in late March of this year, so I wouldn't expect the cables and/or housings to be worn out. I just spoke with another bicycle mechanic who thinks it may be caused by a bit of slack in the cable as the rear suspension is compressed. The loss of tension in the cable would explain why it only happens when I'm riding the bike, and why it only occurs when downshifting and not when up-shifting. He suggested turning the barrel adjuster the next time I'm out on the trail to see if the problem goes away.