Fender supersonic loop

gordiji

I've noticed when using delay in the loop on the clean channel there's an occaisional percussive sound coming through. The vol is about halfway up so already clipping which does cause the same noise but in a more regular way.(as one would expect but also quieter) The repeats on the delay aren't as loud as the initial note, so it's not that. It can sound on the high strings as well as the low strings so doesn't seem to be linked to frequency. The loop is footswitchable so it's easy to hear it's related to the loop.
Doesn't happen on the drive channel presumably because of the increased compression.
External speakers make no difference.
Any thoughts on this ?