When I render a song where the cursor is stationed late in a pattern then a quick audible interpolation on automated parameters from that position to the first line of the song can be heard on the beginning of the rendered file.
Recreate it by setting up an effect that change the sound significantly like a distortion plugin from min to max in the automation editor. It can occasionally be hard to hear so keep trying maybe another effect as it happens with most if not all.
Play the song and stop playing when the distortion/effect is close to max, now wait a second and press play and you’ll hear a ~100ms transition that is happening, this in itself is not a big deal but the issue is this effect is being rendered which it obviously shouldn’t.
If you instead go to the first line of the song before rendering then it will be fine, so the question is why isn’t Renoise telling the render function to go to line 1 and set the initial settings properly before rendering it out?