I have the Onkyo 501 receiver. I know this receiver gets fairly hot during extended use, but I now have it in an open rack. I did have an enclosed cabinet before. Now that it is in the open it does not get as hot as it used to. But now I hear a cracking sound coming from the receiver. Is this just the parts heating up and cooling down? I hear this cracking sound as I use the unit and then after I shut it off. I suspect it is just the unit heating and cooling as it is used and shut off. I was wondering if this is something that is normal across the board with all receivers or just this one, or do I have something wrong? Everything works as it should and nothing seems to be wrong, just the noise.