I probably should post this over at the Cockos forums, but thought I'd post here as well in case anyone can offer some insight.
I've used Reaper for recording audio in the past, but usually I just have an entire MIDI sequence for the entire track, so I record any synth audio start-to-finish.
This time I wanted to record a drone sound and just loop it. So I recorded a short 8-bar loop. But there was a tail at the end of the loop, say an extra bar. So I simply truncated the loop down to 8 bars, then hit "glue items" to get rid of the tail. For some reason, this introduced an audio pop at the end of the loop.
But, if I copy and pasted the loop, truncated the first one down to 8 bars and then glued the first and second loop together (so 16 bars total), I would not get a pop. Then I can reduce the 16 bar loop down to 8 bars, "glue" that one and still no pop.
So at least I can make a loop using the "glue items" feature with no pop, just with a few extra steps. But I'm wondering why truncating the initial loop and "gluing" it by itself would introduce this pop at the end. I should also note I tried this several times with a couple recordings, so it's not just this one recording.
Misc details: Reaper v4.13 (latest), audio card is M-Audio Profire 610 w/ latest drivers, 32-bit Windows XP w/ 4 gigs (3.2 usable) of RAM. Audio recorded @ 48 kHz, 24-bit.
Anytime there's an abrupt start or stop to audio signal that forces an 'instant' transition from -inf dB (silence) to audio signal, or vice versa, you'll get a 'pop' of the magnitude of the audio signal. It's the 'ultimate' transient, in a sense, a vertical line.
You can eliminate the pop by fading up from -inf dB to the full signal (or fading down in the case of cutting to silence) or, in other situations, by crossfading that signal with another.
In the case of your edited loop, I'd assume that your butt splice (loop end to loop beginning) had a small enough abrupt transition that whatever pop there was was not heard.