I recently received an Ender 3 Pro. I followed instructions in a video while assembling the printer. One step indicated that while installing the x-axis belt pulley, to align the captive nuts with the narrow edges parallel to the channels in the extruded rail, which I did. Yesterday, after shifting the printer on my desk, I observed the same symptoms as the OP; growling/grinding noise apparently emanating from the print head during x-axis movement. Upon closer examination, I found the pulley was not fully secured to the rail, and the pulley could be shifted up/down 20 degrees or so; this resulted in the toothed surface of the belt to ride on the edge of the rail, causing the sound. When I removed the pulley assembly, I noted the shape of the captive nut fit the configuration of the rail channel, but only if I aligned the nut PERPENDICULAR to the channel, not parallel. After doing this and re-attaching the pulley assembly, the sound was gone, and the assembly no longer able to shift up and down. This may be a common problem, as the video I referenced is a very popular one. Your mileage may vary.