Cracks and pops without buffer xruns might indicate some interrupt related problem. A long time ago in another galaxy we had a problem causing a very small but very regular data loss over local network communications using protocols with no flow control. Of course we would expect some data loss since we were using the wrong protocols for the job (though on a LAN it's not that common), but its regularity made it worth investigating, and in the end I discovered that an interrupt fired by the video card driver (IIRC) every x time would clog the system for a tiny fraction of a fraction of second during which the CPU would essentially be loaded at 100%. To get to the culprit I had to match to the millisecond timestamps of system interrupts with those of data losses. (I totally miss those days...)

Back to the point: had I played audio instead of transmitting data, I would likely have heard those milliseconds of overloaded CPU in the form of pops and cracks, so it may be worth checking if you have something loading the system in bursts during which it cannot process audio safely.