I often find emergency vehicles don't use their sirens until it's far too late (for me), i.e. they approach a junction with their lights flashing but don't use their sirens until they are within a few metres. It's fine for cars which have generally good all round visibility, but if you appear and pass me from behind within a few seconds as a cyclist, I'll be buggered if I have had a chance to see you at all.
They also seem to like blasting the siren just as they overtake me. Even when I know they are there it makes me jump/swerve.
I understand that I am only a small part of their whole journey - but I don't have an equivalent negative experience whilst driving.