I was thinking there was something wrong with my bike due to the droning when I followed too closely to another Valkyrie. It sounded like a bomber mission from the old 12 O'Clock High TV show.
Exactly THIS!!!
My very first Valk group ride, maybe a third to half of them had Cobra pipes (or just everyone around me).
I start hearing this Waaaa-ooooo, Waaa-ooo, which gets slowly faster in repetition, then slows again.
I really thought my bike was about to blow up.
I'm looking all over the place down at my bike when we pull to a stop, a guy asks what's wrong, I tell him, and he starts laughing. (Except for Air Flow tips, my exhaust was/remains stock on that bike)
It's happened again of course, and it always seems like all the droning bikes get synced up together like their playing from the same sheet music.
The swarm of the big fat baritone bumblebees.