Hi - I’ve noticed some occasional noise on the Monitor output when switching between PFL and AFL modes. I wouldn’t be surprised to get something like a click or a pop when doing this with one or more channels solo’d, especially if the PFL and AFL output from any of the channels isn’t exactly the same (due to plugins, or faders not at 0, for example). But I get this even with nothing solo’d, which seems odd. Also the noise itself is a bit random, and quite often sounds more like a crackle – it reminds me of the sound of dirty switch contacts on an analog console. 
It’s fairly random - perhaps ~20% of the time overall, but that’s a guess - and usually worse on the first AFL/PFL switch after startup, or after some period of time. Usually the noise on subsequent mode switches decreases, and is gone (or inaudible?) after perhaps 2 - 4 of them.
Before opening a bug report, I thought I’d see if anyone else has noticed it, or if it’s just me…
Most recently seen on Ardour 4.4.463, on Fedora 21 + CCRMA, RT kernel. JACK 0.124.1 (and qjackctl reports no xruns or anything unusual around the time of the noise).