Popping noise when I masturbate

This has been the case for as long as I can remember... and I want to make sure it's normal.

More often than not, when I masturbate there is a soft pop noise (like the sound you get when you click your tongue-- except quieter). This occurs with the downward strokes. It does not happen if I make very slow strokes.

I don't think it happens during sex. However, it has happened before when getting jacked off by my girlfriend.

Is this normal/healthy?

Edit:

I think it might happen when the downward strokes cause my urethra to open more. Has anyone heard of this happening?