That's probably the blower wheel. When it slips on the motor shaft, it will make a growling noise. Could also be the support rollers in the back of the tumbler. They will often make a rumbling sound when they're worn, but if they go too long like that, the shaft they rotate on will develop flats, and the rumble turns into an awful growl. Pretty good idea to replace all of the rotating parts (rollers, idler, and belt) together; this is merely preventive, as long as you already have the cabinet open.