My oil fired boiler has started whistling while running. The
high-pitched whistle starts about 10s after fireing up, stays pretty
constant while running and then tapers off after the boiler
extinguishes itself.

Any suggestions as to what might be wrong? I've seen various
suggestions around a build up of limescale, but they seem to normally
be referring to a much lower pitched rumbling sound.

Thanks,
Piers