It's not toads. Not regular ones, at least. Toads whistle to find mates but it's more of a reverberated tremolo than a whistle and you definitely wouldn't confuse the two. My wife and I had to relocate over 50 toads that had bred in puddles on our pool cover because that many toads, all calling every night, was like a coach's whistle hooked up to an amplifier. We couldn't sleep at all. So, we caught every last one of them, drove to an isolated pond 5 miles away and dumped them all in. We haven't had the problem since. The Gray Tree Frog also has a trill tone, but it's call is so powerful that from a distance, it can sound like a spooky nearby whistle. The Spotted Owl has 13 different calls, one of which has been described as an eerie whistle. If the pitch has a sudden upturn near the end, this could be your culprit. It really depends specifically what your whistle sounds like. Good luck finding your answer!

if it happens shortly after the fixture is turned off after being on for a time it may be something cooling down (incandescent bulbs produce some heat). I seriously doubt its an electrical problem.

It does indeed sound like it needs a new drain pump.

I have a Miele W1113 washer. When the drum tumbles in one direction, all I hear is the clothes thumping against the drum sides. But just recently I noticed that when the machine reverses its tumble direction, it makes a kind of higher pitch whirring sound. It only makes this noise in one direction, not the other. Any speculation on what might be going on?