I’ve installed 4 XL systems sound decoders in n scale kato locomotives. And two mrc sound decoders. They’ve all worked perfectly. Some atlas chassis have very TIGHT slots in the frame and if the board is too thick it won’t fit at all. You need to custom add that solder or other shim (Atlas often uses small brass clips) to ensure the board contacts the frame. Also, These sound decoders are VERY sensitive to loss of rail contact, and need to essentially reboot from scratch every time they hit a piece of dust or the tiniest dead spot. They really need a capacitor keep alive, but there is so little space for one that they just omit it and as a result, they are not much fun unless your track and wheels are completely clean and your points impeccable. They also often have some momentum programmed in and take a while to get moving after throttle up. Most of my Atlas locomotives make grinding or buzzing sounds of one kind or another. Sometimes it sounds like the motor is actually contacting the sides of the shell because the noise seems worse when the shell is on. But it could just be amplified by the shell somehow. I have found that the motor in the motor cradle can get slightly twisted and throw off alignment causing noise. Also, the worm gear holder bushings must be perfectly aligned in their “pockets”s and sometimes turning those blocks 90 degrees fixes that issue. So many possible causes for buzzes. It’s why I mostly buy kato locos. 😊👍👍

That’s a 539T not a 251C. But, quit while it runs well enough.