Bongo Fury

I would say it sounds like the glow plug relay is "chattering" as a result of the battery voltage being low. Relay may even be on its way out

must take em a long time to pack up then, i must say that you have to listen really hard to hear the buzz in ours which goes off when the glowplug relay stops with a "clunk" sound.

Do you reckon it could be a dodgy earth..?
I find if anything buzzes on the Bongo.. Clean/tighten up the earth connection.. (in one case a chaffed wire). & no buzz.!