Random pop sounds in my system are scaring me

Hi, I need some help and expertise on this potentially expensive situation! The last few times I've powered up my 18wpc SET amp it has been plagued by some startling "pop" sounds at random intervals. The amp switch is in the mute position, with no source selected and yet at random intervals I hear an alarming pop sound coming from the speakers. The volume of the popping isn't SUPER loud, but it does startle me and has me worried. The weird thing is, sometimes it happens, sometimes it does not. I'll have to do some more testing, but I haven't noticed this happenig when a source is actually playing through the speakers. When I bias the tubes immediately after hearing a pop, I don't see any weird numbers or behavior on the bias meter. The amp is only about 3 months old and it took so long to get, I'd hate to lose it! I know it could probably be a million different things, but does anyone have an idea of what might be happening here? Help!
Thanks!