Wierd sound when taking off standby

[greiswig]

I'm just wondering if this is normal. When I have had my D'Lite on standby (no high voltage at plates) for more than a second or two, then flip the switch to the "play" position, I hear an odd sound. It's pretty faint, but...

In the space of about 1 second, there's a whistling noise very high in frequency that drops in frequency down to pretty low (under 1kHz), then rises back up and goes away or goes up so high that I can't hear it anymore.

I assume it has something to do with capacitors charging, because if I have a voltmeter hooked up to the bias taps, the sound is gone about the same time that the voltage plateaus.

Is this normal? Could it be a precursor to a problem like oscillation? Part of why I'm wondering is that I still have an issue with one of the KT-66s in this am having red plated twice, both after the amp had been on for over an hour, but that issue seems to be so intermittent that I literally haven't been able to force it again. This sound happens no matter what the output tubes are, though.

Thanks as usual!