If you're going to the extent of having to pull the wheel, caliper and or rotor in search of a random noise gremiln; you might want to have a new wheel bearing ready to go.
This is the exact same description - whistling - that the '98 went through last August. I couldn't even hear the sound but our son kept asking about it. It would go away when braking and or cornering (but I forget now which way I had to corner)..
In the end, the PS rear bearing was toast and ultimately that random, hard to hear, whistling sound became a constant that I thought for sure was a brake caliper rubbing on rotor or some bolt stuck "in there" sound... Luckily, I had a set of bearings for the rear on hand.
Surprisingly, a wheel bearing is relatively easy to replace compared to any other work I've done to the '98 or '00 since.