From what I've read this is a common problem with looping in general especially if you're trying to loop a continuous sound. The start and end points might not line up the actual waveform of the sound so there may be a little hickup. The start point starts at one point of the waveform and the end point is at another point and then when it goes back to the start point it's as if the waveform suddenly skipped a part. I hope that makes since.

So for the sine wave if the start point is at the top of the wave and the end point is at the bottom when it loops, then it makes a jump from bottom to the top without anything in between. So theres a vertical line in the waveform like in a square or sawtooth wave.

I dunno if that's the problem or not. Maybe that wouldn't be noticeable.

For looping on the sp-555 I like the pedal and measure features the best. Figure out the tempo and then start it on 1, record for 1, 2 or 4 measures and then it automatically stops. For a drone I would make sure you've already got the note playing before you hit record too. I'll try some stuff later tonight

Yeah, you are right about the waveform and how that would cause a hick up in the sound.

Well I tried situations where a note was already playing and then I hit record. It still had a little gap in it. I'd be curious to see the actual waveform though. I tried it with some low notes and I wonder if it's as noticeable with high pitched sounds.

If you want to see the waveform, just record your loop with a free program like Audacity to your PC.

Did you try looping one monotonous sound (keyboard string that has no variation in it)? If that also causes a hick-up then something is wrong...

Hmmm. There appears to be a small gap at the end of the waveform when I loaded it into Cubase. Also some at the beginning. About .008 secs at the beginning and about .02 secs at the end. That would be the problem then