[raymo]

Thanks for the suggestions JA. The only thing that I haven't done is remove
the cover, because I am no longer stable on a ladder without holding on to
something. Getting old sucks. Looks like I'll have to bite the bullet and get
it professionally looked at.
Incidentally, I think you are right about the cause,
because as far as I know there are no moving parts inside the unit, apart from the fan
and the vane swing feature which is turned off. I discovered that the whistle doesn't
actually stop between pulses; it just becomes almost inaudible.
raymo