I've been have a very similar or possibly the same problem. At random times, whether playing audio or not, I will start hearing a popping noise. If I'm playing audio, it cuts in and out. If I'm not, I just hear the light popping. My speaker icon will fluctuate from whatever level to (apparently) full. If I click the speaker icon to get the volume control, it goes from enabled at whatever level to full and grayed out. If I go to System Preferences>Sound>Output while this is happening, the output will jump from Internal Speakers to various other output options in a seemingly random order.
This only started happening after one of two events but not immediately afterwards: 1) The release of 10.5.3 2) Upgraded hard drive.
I can't say exactly when it started, but it was definitely a short time after those two events which occurred within a week of each another.
Further, this doesn't happen every day. It seems to happen a couple of times a week at random times. It will last for anywhere from a few seconds to as much as a minute and then cease.
For sake of showing that I haven't lost my marbles, I even made a video of this strange happening and have posted it here:
http://www.g2web.com/mac/Mac-Audio-Weirdness.m4v
What's odd is that the audio from iTunes all recorded perfectly to video while all this was happening even though what came out of the speakers cut in and out.
Any ideas/help would be greatly appreciated.
Thanks! 🙂