Artifacts present in the harmonica preset of the SF3 version of the default soundfont.
The problem is outlined here in this support form thread....
At first I thought it was a bug in Fluidsynth, until I realised I was (of course) playing the SF2 version in Viena.
Werner's SF3 encoder is again introducing artifacts into some sounds - I currently have a similar problem with the trumpet, which is why there is no new version of FluidR3Mono_GM.SF3 uploaded,
Initially I am going to try a recompile of the SFtools encoder, but if that doesn't work someone may have to look at the code again, which is way beyond my understanding!